A fat, angsty, transgendered kiwi teen? This is the plot?

I would have enjoyed this movie more if they had killed him and spared the pedophile.

I thought the kid was a great character, the young actor who played him was terrific, his story was well-written, and the kid was a great foil for Deadpool himself.

The thing about Deadpool is that he's a very limited character, his emotional range consists of about 90% snarkiness, and 10% romanticisim. So while he's fun to watch, if they want the film to have any depth... all the range of feeling has to come from other characters. So, they brought in a genuinely tragic character, the mutant that's despised by not only mainstream society but other mutants, and who's doomed to turn evil... and had him do an entire film's worth of emoting. The kid actor did a good enough job that you can believe that for once in his life, Deadpool would drop the snark and genuinely sympathize with another human being.

IMHO a good superhero movie needs a good balance between action scenes, fun scenes, and dramatic scenes - the dramatic scenes are necessary to give stakes and suspense to the action, and some fun so we enjoy the movie overall. Now while Deadpool is great at both action and fun, he's not much for drama once his initial shock at becoming a butt-ugly superhero was over with, so the kid did a great job of providing the drama that Deadpool cannot.

I think the technical term is "foil".

Hated that actor.

And this is the weakest of the trilogy.