Hello - The Humbug character in the deadpool set has an ability "I can command insects with my mind" which allows him to choose an opposing character in hindering terrain and counter one power the character has.

My question is, if the character who is in hindering terrain has stealth, can it still be the character selected for this ability, are does it’s stealth prevent it from being selected?

I would say that since the power doesn't mention needing line of fire or uses the word target then you could use humbug against a figure with stealth or any game effect that mimics stealth

Short Answer: Yes, he can use I Can Command Insects to target a character with Stealth in hindering terrain.

Longer Answer: Since ICCIWMM!W,IWAF states it targets a character within 6 squares, and it doesn't require Line of Fire, then Stealth would never come into play, since LoF is never drawn.

The wording just says "choose" and makes no mention of line of sight or targeting so you should be able to.

I agree with this. Because it says, "choose an opposing character occupying hindering terrain within 6 squares" and doesn't say anything about LOF, Stealth doesn't come into the equation.