I can see him becoming a monster, though, if he eats too much mana too quickly. Think Murder Mystery from Mystery Ben's Mystery Skulls music video series (which also features a good-aligned kitsune named, well, Mystery).
Oh wow, flipping through that gallery, I love that character design. Especially the dog form, ha ha
Mystery's backstory is actually a pretty good one. Apparently, like you, Ben did his homework when it came to kitsune mythology.
Over a millenia past in the wilds of ancient Japan there lived a rare and dangerous beast: A nine-tailed
yako-kitsune, wild, untamed and imbued with the power of the kami, the Shinto gods. A samurai warrior, Mushi Yukino, went out to face him as he was causing a lot of trouble in the neighborhood. What exactly happened in those ancient times is a...mystery. But as far as we've been shown:
1) Mushi defeated the
yako-kitsune in a furious battle, using a magic-imbued blade blessed by the god of foxes, O-Inari, Herself. The battle stripped two of his tails, reducing him to a
nanao no kitsune, a Seven-Tailed Fox.
2) As a result of his defeat, he was given a choice: Death or binding to the Yukino clan until O-Inari awarded him the title of
zenko-kitsune - Noble Fox. Traditionally a
yako-kitsune is bound for five hundred years of servitude before being allowed to join the
zenko. Thus, Mystery has been a part of the Yukino clan all this time. Vivi is a descendant of Mushi, so Mystery is still bound to her as her guardian.
3) The battle was quite bloody, and some of his Mystery's blood leaked onto the roots of a cherry tree, which absorbed it. The tree became the demonic yōkai Shiromori, or the White Tree Demon. She attempted to help her "father" in the battle, but when he bound himself to the Yukino clan, he abandoned her. And she was
pissed.
4) Shiromori has been chasing him ever since, for all 1000 years he's been guarding the Yukinos. She is thirsty for his blood, as it is what gave her life, and powers her abilities. The ancient demon can form flowers based on the spiritual traces of people she can find, and use them like compasses to locate them. She is a
nasty piece of work.
"You can't run forever, mutt."---
It's always been a major curiosity why he chose a small bob-tailed dog as his primary disguise. Most kitsune disguise themselves in human form, and they
HATE dogs - the one animal they have no power over. But Mystery is a very quirky zenko-kitsune.
He's also, of course, meant to be a parody of Scooby-Doo, same as the entire series. As he was once a
kyuubi no kitsune, that makes him...
Kyuubi-Doo.