"...Alex? Its me....I'm sorry...."--Hannah
At the end of Night 5, snapping of a neck can be heard in Melissa's last call, presumably from Mr. Handy. This is also the night Hannah becomes active and tries to kill the player. After the events of the Inventory, she got her own attraction; Sound Valley. She now performs by day and kills night guards by night.
Hannah acts on two sides; As she moves, Melissa's body moves. Melissa disables everything you have, and Hannah acts as a beacon for the animatronics. They become active in Nights 4 and 5.
Generally, she is very optimistic and sadistic, with a smart alec attitude.
Alex: Hannah has a vow not to kill Alex, since she tried to help him survive; she tries to kill Alexin FNATInvent due to peer pressure from animatronic's threats.
Merle: She hates her and boycotts her. This is because of Merle's selfishness, of which she doesn't like.
Lindsay: They don't want to see each other; as they both tried to kill each other. If they ever talked though, they would realize that they get along fine.
Mr. Handy: She has another vow; to rip Handy apart, since he killed her. She needs help to do this, though, and is finding people to aid her.
-Sound Valley is a show stage set in a desert at night, with a pickup truck Hannah sits on. This scene was inspired by Meet The Engineer.
-Melissa's original final call hinted at being stuffed in Hannah; "Its.....me....Hannah...."
-Sometimes, a garbled mix of static, a unknown love song, and screams can be heard while Hannah is in the kitchen.
-She is puppetered by a unknown animatronic; a theory is that its a hallucination Melissa had that came to life.
(Credit to MakaKishinKiller for the picture!)