He is one of the two prison cell guards, along with Krupper, and Sarah was once Flip's lover.
Interactions with the player character
Involved in quests:
Unlike Krupper, the Vault Dweller can speak with Flip even if he is not wearing robes like the Children of the Cathedral, although he will immediately attempt to escort the player to the Lieutenant. If the player is wearing the robes, he will tell him to go to the control room. Failure to comply will result in him attacking you.
If the player enters the cell that holds Sarah, Flip will scream, "Don't you come near her! I'll kill you first!" It is very clear that Flip and Sarah have a romantic connection as a product of him watching over her. If the player kills Flip, Sarah appears very upset.