This page always bother me. Why did Yuma insist on this? He had no way knowing Osamu would been a good captain. I think its two reasons

1) Probably used to doing things solos so he didnt want to bother
2) He knew Mikumo was both weak and unskilled, so he needs something to pull his weight on this team. Had Yuma been the Captain people like Osamu would get crushed on their uselessness.