The "entity" Jacob acts very much like a wraith. Made famous by Tolkein a wraith does not have their own free will. Jacob could have been the whole reason the Dharma group was there in the first place. When key members of Dharma(Richard) realized what was being experimented on they broke away and became the hostiles. Richard or Ben figured out how to "trap" jacob in a magic ring of gunpowder-look for someone to light the smoke monster on fire to get it to go away. The problem with having the wraith-someone controls it. It is possible that the sounds of horns heard in the jungle before the monsters appearance was the wraith's master bidding it.
Pros: Seems to be at the least one of the key "powers" that a number of characters connect to (i.e. Widmore, Hanso, Paik).
Cons: Charles was in Scotland just before Desmond left for his race and crashed on the Island. Desmond most likely learned about the Others, whom he called "hostiles", through Kelvin - which would suggest the Others have been on the Island for a long time. Thus, it would make sense that Jacob has lived on the Island many years before Desmond arrived. Jacob lives on the Island, as shown in "The Man Behind the Curtain".
Conclusion: Considering Charles's stature in the "real world", it is doubtful that he resides on the Island with the Others.
Cons: In the Lost Experience, Alvar is in Norway in September 2006, and says he has been held as a prisoner in his home by Thomas Mittelwerk. Plus DHARMA and the Others are enemies.
Conclusion: Alvar Hanso would seem like a reasonable candidate for "Him", but factual information from the Lost Experience does cast some doubt on the theory.
Alvar Hanso's Father
Pros: We have not yet learned the name of Alvar Hanso's father, who would also be Magnus Hanso's son, which leaves open the possibility that his name could be Jacob Hanso. Given that we don't know the nature of Alvar's father, it's possible that part of the DHARMA Initiative's purpose on the Island was to discover what had become of Jacob Hanso.
Cons: Jacob Hanso would probably be at least 90 years old, which the vision of Jacob in "The Man Behind the Curtain" suggests he is not, unless the Island has slowed his aging down considerably.
Conclusion: The missing Hanso between Alvar and Magnus seems like a strong possibility to be named Jacob Hanso, but there is no other evidence that he is Jacob except for the fact that we don't know anything about him.
Pros: Due to the regenerative powers of the Island, Magnus might have lived this long and taken on some of the stranger characteristics of the Island as a result. Any resemblance to Alvar Hanso could be explained. We don't know if Magnus has a middle name. Jacob hates technology like a flashlight, but oil lamps are fine. Jacob's house has chains hung throughout it, like the Black Rock.
Jacob's clothing in the screencap appears similar to that of the young Richard Alpert, and may be 19th century in style.
Cons: Magnus is probably dead, and his "confirmed final resting place" is a part of the blast door map. Although, the final resting place could very well be his cabin.
Pros: Information from the Lost Experience suggests Mittelwerk, who took over as the Hanso Foundation leader, has a similar ethic to the Others; willing to kill in order to obtain some "greater good". Both he and the Others seem to be motivated by an "ends justifies the means" school of thought.
Cons: Mittelwerk is present in the "outside world" during the Lost Experience, and seems to have taken time establishing himself in the Hanso Foundation. Thus, its probable he wouldn't be on the Island in relation to the Lost time line of late 2004. Jacob lives on the Island, and Mittlewerk wouldn't hate technology the way Jacob does.
Conclusion: Mittelwerk does not seem the most likely candidate for Jacob due to chronology, but similarities between he and the Others is striking, suggesting Mittelwerk could somehow be connected to them on another level.
Pros: Enzo apparently died in a plane crash, but could have actually survived and disappeared to live on the Island. As the founder of the Valenzetti Equation, his research sparked the DHARMA Initiative itself.
Cons: Valenzetti may have actually died in his plane crash, and so could not possibly be Jacob. Besides that, the plane crash was in Europe. Also, Rachel Blake's evidence in the Lost Experience suggested Valenzetti was in hiding in Italy. There is also no reason to believe that the leader of the Others is the man who, in essence, spawned the DHARMA Initiatve through his equation.
Conclusion: This theory seems perfectly plausible, as Valenzetti's importance to the Island's mysteries (e.g. the Numbers) seems quite profound.
Pros: Kelvin could have survived Desmond's assault, being found by the Others, where he became a leader figure with his extensive CIA and DIA training knowledge. Alternatively, when leaving Desmond in the Swan, Kelvin could have been returning to the Others' camp to pass on information.
Cons: It seems doubtful that he could have survived the force at which Desmond hit him against the rocks. Kelvin also worked for DHARMA (the enemy of the Others), seemed depressed about his life in the Swan and there seems no simple explanation for the 3-year deception he would have had to have played on Desmond. It was also mentioned in an Official Lost Podcast that the Others were not aware of the inner workings of the Swan station before the crash of Flight 815, which they almost certainly would have had Inman been passing on information.
Conclusion: The idea behind this is valid, but evidence would seem to suggest that the latter part of the theory at least is unfounded, whilst the former is rather unlikely.
Pros: In "Stranger in a Strange Land", Jack is told in a flashback that he is a "great leader". At the end of the same episode, Isabel tells him that the Chinese symbols on his tattoo mean "he walks among us, but he is not one of us", but Jack tells her "that's what they say, but it's not what they mean". The expression on Ben's face as he looks at Jack while boarding the boat seem to indicate some reverence toward Jack. Jack's name is derived from Jacob, and the fact that he "wasn't even on Jacob's list" may indicate that Jack and Jacob are one and the same.
Cons: 'He' was presumably already known to the Others before Flight 815 crashed, so it seems unlikely that a stranger such as Jack - who Ben admitted they were trying to "break" - would be Jacob. Ben's apparent reverence toward Jack may be due to his being grateful that Jack saved his life in spite of them being enemies. Jack was also beaten up by Ethan Rom in "All the Best Cowboys Have Daddy Issues", which wouldn't strongly suggest that he is not their leader. Jack was with Juliet when Ben and Locke met with Jacob.
Conclusion: Overall an unlikely candidate, since he seems to be unfamiliar to the Others (aside from them knowing things about him like they know about everyone else on the flight), and due to his confrontations with Ethan. However, the name coincidence (Jacob) and his designation as "a great leader" are good evidence for the possibility, as is the fact he refused to return to the Island with Kate and Sawyer.
Pros: Given the possibility of cyclical time on the Island, Locke could also be Jacob. In this theory, Locke would need to be much older as Jacob. The potential would exist for a young Locke to meet up with an older Locke (Jacob) at some point. The biblical Jacob was a trickster, and Locke tricked Sawyer into killing Cooper. Locke was asked "are you Him" by Desmond, to which he replied yes. Locke has a special connection to the Island, which, if a cyclical time situation exists, would give him unique insight. Also, Locke has stated he has no plans to leave the Island in the event of rescue. In addition, comparing Jacob and Locke's portraits together reveal a stunning similarity.
Cons: The theory requires acceptance of a cyclical time stream on the Island. Locke refused to believe that Jacob exists in "The Man Behind the Curtain". Also Ben shot Locke at the end of episode 20 which puts more weight on the probability that he is not Jacob.
Conclusion: Overall, the cyclical time stream may yet be true, but Locke is probably not Jacob.
In "The man behind the curtain" Jacob's profile reminds me exactly Locke. Jacob could be Locke's brother (maybe "Twin") or Locke's alter ego. The "eye" we see in Jacob's room is almost BLACK: it reminds me of Claire's dream in the first season, when she sees Locke with a black eye and a white one.
Pros: Peter Coyote is the Lost narrator for recap episodes, leading to some regarding him as the "all seeing eyes" on the Island, a quality Jacob might exhibit.
Cons: Jacob could be someone we have previously already encountered in the show.
Conclusion: There is little one can argue with this theory when there is no actual reference character, but it could certainly be possible that Coyote will be one day cast as Jacob.
God
Pros: Consider: "If I tell you about them, you don't know what he'll do!", "He is a great man, a brilliant man, but not a forgiving man.", "He'll kill me!", "He" will kill him for failing his mission. Also, Ben continuously states that the Others do not kill - one of the ten commandments
Cons: Ben asks Jack if he believes in God. Hinted to the plane crashing with a spinal surgeon two days after he was diagnosed. Ben's mission is to get Jack to perform the surgery or Jacob will ultimately take his life through Cancer.
Conclusion: Seems like a bit of a stretch. Ben has shown no other likeness to being religious. Additionally, many traditions hold God to be forgiving. Besides, even if "He" were not to be divine, he could still infect Ben with cancer, possibly via radiation.
The Devil
Pros: Consider: "God loved you as he loved Jacob". If Jacob is not God, then he may be The Devil - an angel formerly loved by god, but cast out from heaven after falling out of favor. The Devil, as opposed to God, is not forgiving. Ben told Locke that Jacob is one who summons you; he may have meant this in a spiritual sense, as Jacob may be summoning the spirits of the deceased, thus all the sighting of Yemi, Christian, and other supposedly deceased persons. If the Losties really did die in the Oceanic crash, perhaps Jacob/The Devil Summoned them back to life, or at least to the Island. The 'circle' around Jacob's house may be part of the means to entrap The Devil on this island. Still, Jacob can occasionally tunnel under these entrapments, reaching out from underground with the 'black smoke'. This explains why The Smoke originally tried to drag Locke underground - he was being 'summoned'. If Jacob is the Devil, he seems preoccupied with the 'good' people on the Island - perhaps he is trying manipulate these people into becoming 'bad' by breaking a commandment - like killing your own father, much the same way The Devil wishes revenge on his Father. The Devil has been trying to find permanent escape from his prison, and has been manipulating Locke since the beginning, asking him to 'Help me' all along.
Cons: Like the God theory, a little far-fetched. The Devil would not help heal Juliet's sister, improve the Island inhabitants' sperm count, or condemn Eko for failing to repent for his sins. The Devil would also likely not be affected by the sonic fence.
Conclusion: About as likely as the God theory, unless both are true. There are many references to a battle between 'dark' and 'light' forces, and Lost characters have reported the Smoke as being either 'a bright light' and 'beautiful' or what we see.. a dark, menacing cloud.
Pros: Has the ability to heal inhabitants (e.g. Locke, Rose), and could, as a commonly held fan theory suggests, punish survivors for their sins. It's human form as seen in "The Man Behind the Curtain" could be a figment of the individual who is seeing it's imagination.
Cons: The idea that the Others' leader is an inanimate piece of land seems very implausible.
Conclusion: The Island is undoubtedly unique in a number of ways, but too many references have been to Jacob being a man rather than an abstract concept.
Cons: Evidence seems to suggest that the Monster is a "security system", perhaps mechanical, so is probably not Jacob himself.
Conclusion: The Monster itself does not seem likely to be Jacob, but so little is known about it that it is possible it could take human form. It could also be argued that the Monster is controlled by this mysterious individual.
In "The man behind the curtain" Jacob's profile reminds me exactly Locke. Jacob could be Locke's brother (maybe "Twin") or the Locke's alter ego
Other ideas
Jacob is a completely new character who we haven't yet met.
Jacob is some sort of disembodied spirit seeking a body to inhabit- hence The Others interest in pregnant women and babies.
Jacob is a psychic character with powers similar to Walt, and can judge people like the Monster.
"Jacob" is the last name of a character we have already met.
Jacob is the central human deity of the Others' religious cult, analogous to Jesus, Moses, Muhammed, etc. Jacob's list of 'good people' was actually made hundreds, if not thousands, of years ago and is a prophecy. This religion has been waiting for the 'good people' to arrive for hundreds, if not thousands, of years, and the Others believe the 'good people' have been sent by divine intervention on Flight 815. In Jacob-mythology, there is a parable in which Jacob lost a toe for some reason.
Jacob is the author of a list of prophecies describing future events to be experienced by the Others. Jacob is a great man (he predicts that the Others will do great things), a brilliant man (his prophecies are especially precise and detailed), but not a forgiving man (the prophecies must be fulfilled exactly lest disastrous consequences occur).
Jacob's name is another reference to the Old Testament, specifically Jacob's Ladder. Jacob is revered because he can communicate with Angels or spirits.
The title of the episode "The Man Behind the Curtain" seems to make reference to the Wizard of Oz. Ben could be the man behind the curtain, using the idea of Jacob to control The Others and remain in power, just as the wizard does to the people of Emerald City.
He is a healer -- and considering the line of Wizard of Oz references, it would be easy for a skilled and charismatic surgeon to convince people in need of leadership that he is a mystical healer.
Jacob is a member of the crew of the Black Rock, possibly the captain. His clothing appears to be of that era. This could also explain his aversion to technology.
Jacob's non-corporeal state was caused by Dharma's incident, hence his hatred of technology.
Jacob is a psychic manifestation of Ben's split personality.
Whoever sees Jacob sees him as themself - this is why Jacob looked like Locke in "The Man Behind the Curtain".
Only those who believe in Jacob can see him.
Ben says it is unfortunate that Locke is too naive to see him.
Locke only sees Jacob after hearing him. He didn't believe that Jacob was real until he heard him.
Jacob is a hallucination brought on by the Island's unusual electromagnetic properties. It has long been thought that rogue EM fields can induce some 'hypersensitive' people (in this case Ben and Locke) to hallucinate the appearance of ghosts, angels, aliens etc.
Jacob is a great healer, and the island is where he is most powerful. If he was anywhere else, he would be a lot weaker.
Jacob is a doppelganger of an already-existing character, or vice versa.
Actions
Jacob is being held prisoner by Ben. The line around Jacob's property keeps him there. This is why he told Locke to help him.
Ben is being held prisoner by Jacob. Something happened to Jacob's body rendering it invisable/useless(could be from an extended amout of time on the island or possibly from "the incident"). Because of this, Jacob needs to find people who have a strong connection to the island's properties to use them as "host-bodies" to swap souls with, so to speak. When Richard heard from Ben that he saw his dead mother on the island(just like Locke and Cooper), he realised the strong connection Ben had with the island and saw the opportunity to supply his master(Jacob) with a new body. Directly after the purge, Ben's actions and body language was that of someone who has been a leader for many many years, Richard also treated him in the same manner. When Locke heard "Jacob", it was really the traped Ben asking for help while traped in Jacob's body. Only Richard, and possibly Tom know the truth behind the Jacob/Ben situation.
The list is the Middle Section version of the list Ana-Lucia found on the dead Other, listing details about the tailies in "The Other 48 Days". It may be a list of "good" people for the Others to kidnap, which has interesting implications for Jack.
Sawyer and Kate aren't on Jacob's list either. They are there to con Jack.
We know Kate isn't on the list, but we don't know about Sawyer.
We know Sawyer is a self described bad person.
We don't know that self-described bad means your off the list.
Sun would actually seem to be a good original candidate for the list, but her killing of one of the Others disqualified her by Season 2.
She slept with another man while being married to Jin. Depending on the extent of the Others information and their opinions on relationships, she could be excluded because of this.
Ben killed his father, participated in the slaying of several dozen DHARMA employees, and had Lock kill his dad to become an Other...How whopped is the Other's sense of good and bad?
In "Not in Portland" when Juliet is looking at the monitors, there for a quick flash on a monitor is a shack and the film is slightly fuzzy.
Jacob may control the island's fate. As the island seems to be closely intertwined with greek mythology, 3 beings called the Fates controlled the destiny of the world. Jacob may be somthing akin to that.
Other Theories/Casting
Could Jacob be played by Kris Kristofferson? While there is little evidence (aside from appearance and voice) to support this theory, a number of viewers noted that the voice that said the words "Help me" did in fact sound similar to Kristofferson's unique voice. Additionally, the profile of Jacob certainly resembles that of Kristofferson.
