Gally is the result of an unfortunate and assuredly unpleasant laboratory accident. The foresnsic teams aren't sure of quite what happened and her memory of the event is rather fuzzy, but sufficed to say it involved a gallium/mercury liquid alloy and a bottle of a mutagenic substance. No-one, least of all her is quite sure of who she was before the accident, and given there are several people still unaccounted for, this state may remain for a while.
Gally isn't exactly the most confident of people despite the fact she is, fundamentally, immortal. She doesn't like the fact she is a tailed, only vaguely human freak of nature, she doesn't like the fact she starts to melt above 40 degrees C and starts to freeze not far below freezing and she especially doesn't like having her height or weight mentioned. Other than that, she's pretty much the average teenager.
Recently, she's started to date Ash Phoenix, someone who's practically her opposite. It appears to be being good for her.
She's a blob of liquidy metal. Mostly, this boils down to being stronger and faster than the average person (no muscles to worry about, very dense, etc.). If 'wounded', her body tends to schloop back together in short order, though this is proportional to the damage. An RPG to the chest will tend to scatter her over a fairly wide area, for example, and it takes a while for her to pull herself together. In combat, she mostly relies on her regeneration and strength to batter her way through most problems and isn't really well known for laterla thinking.