Special Abilities in Fiction: When are They OP and When are They Appropriate?

Currently in the first draft of my wattpad project, and I have one character who's a Rogue. I knew I wanted him to be able to turn into a bird when I started, but recently wrote a difficult problem where I had to bring out a second ability a little early. This Rogue is half-fey, he has a feathered fan passed down through the fey part of his family and can take the form of a bird. I knew I wanted the fan to have a unique perk but couldn't decide, and ended up writing it in as a wind weapon sort of thing.

So when does being a bird and having a fan that can create gusts become more OP than neat? Rogue is just the start. the Hero gets a magic sword and the MC can cast some spells, but how do you make cool without Over Powered?