Well... I've never actually met him, personally, but the reports are detailed enough. Adopted brother of Thor, actually the son of the Frost Giants, hates and resents Thor for being a presumed glory hog and took it out on Midgard, AKA Earth, whenever he could. Power-wise, he's super-strong, heals from injuries faster than humans, and his magic can create illusions or make him disappear or shapeshift at will.

[What else is he missing...]

...Oh, and I think he's a girl now.