So first I'm running with the idea from @omniadventerous for aquatic Osmosians.
Because Kevin doesn't have horns, I feel like they may have been removed when he was born.
That leads me to believe if hybrid Kevin had any other irregularities, they would have been corrected by the hospital.
So say if he had a tail, horns or non humanoid genitalia those would all have been 'fixed' in infancy.
And that leads to why Kevin had trouble fitting in as a child, he felt different from his peers
But that's also why absorbing electricity made Kevin go insane because he did not have a tail or horns to help store or regulate his power.
And going further that's why Aggregor had trouble with power madness. Because his tail was removed as he was already a powermad criminal.