The Molotov woman is an Other of the background cast who accompanied Tom and the Twins when they kidnapped Walt from the raft. The significance of this woman and the Others on the boat is that this is the first time the survivors have seen/encountered the Others, excluding Ethan. This Other also stirred up a lot of theories on her identity/who she was portraying (either as the character or as a stand in). She was speculated to be Colleen Pickett, Juliet Burke and Isabel.

Season 1

After a brief conversation with the survivors on the raft, Tom declared they were going to take the boy. After Michael furiously refused and Sawyer grabbed his gun, the boat's lights were shut off, and the Others attacked.

This woman was the one who blew up the raft with a Molotov cocktail. ("Exodus, Part 2")

Season 3

She was later briefly seen in the camp at the Ruins. ("The Brig")

Season 6

The Molotov woman was most likely at the Temple with the rest of the Others, and may have fallen victim to The Man in Black. ("Sundown")