Giant-Size X-Men #1 brings him into the rescue team, and he stays for the new X-Men rather than returning to occasional guest appearances.

Giant-Size X-Men #1 brings him into the rescue team, and he stays for the new X-Men rather than returning to occasional guest appearances. His power is easy to describe badly. He screams. But he can use sound as an attack and keep himself in the air with it, which makes him useful in several different kinds of trouble. The age difference also matters. This isn't another teenager being brought into the school. Sean has a life behind him and has to decide whether joining this particular group makes sense. Then Thunderbird dies almost immediately after the new team gets started. Sean's experience doesn't make that easy, or make being an X-Man suddenly feel like a safe career change. There's a reason he sometimes questions whether he should stay. He's old enough to understand that being able to do the job doesn't mean the job can't kill you.