I watched an anime that I think came out in 2007 possibly, that I can't remember the name for. It was about a young boy and I think he was taking care of his little sister but they think they ruled the city they lived in. They had a fight with some other kids, a lot of unrealistic jumping in the air from buildings and such. Kind of fantasy. I think the boy has some sort of confrontation with a 'demon' within himself.
I remember a part where his little sister found a plant or maybe it was a fish that she was trying to keep alive in this yellow car they slept in, in an alley.
If anyone can help that would be awesome :)

Tekkon Kinkreet
The story follows two orphans, Kuro and Shiro, as they attempt to control and cope with the violence they find on the streets of their city, Treasure Town. Kuro is a streetwise punk, while Shiro is an innocent dope, out of touch with the world around him. First, they take on thugs, religious fanatics and yakuza. And when a corporation called "Kiddy Kastle" conceives plans to tear down and rebuild Treasure Town to fit its own goals, Kuro and Shiro work together to prevent this.

Amazon.com: Tekkon Kinkreet: Kazunari Ninomiya, Yû Aoi, Yûsuke Iseya, Kankurô Kudô, Min Tanaka, Rokurô Naya, Tomomichi Nishimura, Mugihito, Nao Ohmori, Yoshinori Okada, Kazuko Kurosawa, Tomoko Murakami, Michael Arias, Eiichi Kamagata, Eiko Tanaka, Fu (http://www.amazon.com/Tekkon-Kinkreet-Kazunari-Ninomiya/dp/B000TGCR3I)