After the Pacifist Ending, Frisk eventually gave half their soul to Chara, trying to give them a happy ending since they weren't able to give Asriel one (Besides, he refused it). This caused Chara to become permanently corporeal.
Initially, Toriel and Asgore were relieved and overjoyed to meet Chara again, even though Chara wasn't, knowing how much grief they had caused them. Chara didn't forgive themself. And even though years have passed, and they have grown attached to their new friends and family, (even bringing Flowey from Mt. Ebott) they still haven't forgiven themself.
Being made entirely out of Determination (And having one half of a Determination soul, so more Determination), Chara's HP relies on how determined they are, similar to how the HP of Magikin (Monsters) relies on how hopeful they are. Being made out of Determination, they can easily generate weapons for themselves (made out of DETERMINATION!!) and also know fire magic (Taught by Asgore and Toriel).
Oh and they can also turn into a flower, like Flowey. (Probably because of the fact that by the time Frisk had fallen on their grave, a bed of flowers had already grown from it).