Origin: Daniel and Charlotte presumably became friends after being recruited by Abaddon to find the island.
Since Then: After parachuting onto the island, they were separated. Sayid harnessed a deal with Locke to trade Miles for Charlotte, which reunited her with Daniel. Upon imminent rescue, Charlotte decided to stay on the island, telling Daniel cryptically that she was still looking for the place where she was born. Once the island moved and the castaways started flashing through the various time periods, Charlotte started experiencing nosebleeds. When they got themselves captured by The Others, Daniel professed his love for Charlotte to Richard. The flashes caused Charlotte's condition to worsen, and she soon died, but not before revealing that she may have met Daniel when she was a young girl living on the island. During the three years he spent on the seventies, Daniel tried to avoid her, but he finally talked to a younger version of Charlotte in 1977, thinking his plan to prevent the Swan Incident would work.
Origin: Locke took Charlotte hostage while Sawyer was part of his group.
Since Then: The two only knew each other briefly while alive. Sayid retrieved Charlotte from the Barracks soon after she she entered them. Sawyer next met her when the time flashes began. Impatient for an explanation, he even threatened hitting her, and he later angrily demanded she translate Jin's Korean. Charlotte died soon after. In the flash sideways, the two met on a blind date and spent the night together. When Charlotte discovered James's folder covering his search for the original Sawyer, he kicked her out of his house. He later visited her apartment with a flower and some beers, but she rejected his apology. He left the flower at her doorstep. The beers went home with him.