This is Yakumo, from the manga 3x3 eyes,
which in 2003 just ended a very long and successful run. It's
basically a story about this kid who meets this girl who was
friends with the kid's archeologist father. Turns out, she's
immortal, and when things go wrong and he dies, she summons his
soul into her body, thus making him both immortal and her servant,
called a Wu. It's hilarious, especially when he does things like get run over by a bus, There's also an evil teddy bear too.