Ah, yes- I forgot to add the essential details. My bad!
It may seem cliché, but I'm a sucker for stories where the male lead saves the damsel in distress. So in answer to your question, the inflatee would be the hero, who saves me from the villain, and I offer to cook him a homemade meal as thanks, and cute “date” scenarios ensue before I ask about his powers, and he inflates himself. Maybe by inhaling a lot of air. But me personally, I'd like the hero to fill up with water while I lay on his expanding belly, and we can gaze at the stars. (Of course, this doesn't have to happen, it's just an example/fantasy)
Like I said, cliché... ^^'
Possibly... do you want the inflatee to be the hero, or a villain, or a hostage, or an innocent bystander, or, or, or...?