Part of the Governor's second group, went along to the prison and abandoned them when they beheaded Hershel. Glenn rescued her and they traveled to Terminus together for half a season. She's a lesbian. Eugene hit on her once.
Got injured in the explosion in the last episode and is being treated by Pete, hence why Rick is going to get her killed, with no surgeon she is probably going to die, they said as much in the episode. They can't deal with Pete right now because Tara's life is hanging by a thread.
Rick ignored that anyway and put Jessie before the life of one of his own.
I really want a scene in the finale that really addresses this and he's called out on it. Rick's been about making decisions that are in the best interest and safety of the group. However he's been lusting (that's really what it is since they haven't been having any heart to heart talks or spending a lot of time together) after Jesse that he's tossed that away and doesn't even see it. Hopefully that rock to the head by Michonne will give him time to examine his situation and what's been doing.