This is Cekari, affectionately called "Little Half Heart" by some people. She's an over all outgoing person but like many people- she has her gloomy moments- though if someone else is around when she's gloomy, she'll usually shake it off and try to be optomistic. If she gets too close to a keyblade user (or an angel) she will turn into her shadow form and usually track down that person. she also turns into her shadow for when she gets too much power.
#Cekari
(more info in the comments)