Don't understand why so many people are acting like the MC should be arrogant. Do none of you have friends? Should this group just move forward in silence and not speak or try to get to know and understand one another better?
Why risk your life for someone if they won't be a little open with you? If he treated her like you guys suggest, there'd be no reason for anyone to help him with anything. Ever. Best to just leave him on his own whether he can fight or not.
They're adventurers. Knowing your partners is usually a pretty big thing, especially one's capabilities - so that you know what you're working with and how to best function as a team. Hiding your abilities as part of a life-and-death team is some really edgy chuunibyou bullshit, not sensible at all.
He can easily give an overview of his capabilities and ask that no one pry into how or why he can do these things, just let them know what he can bring to the group. He's relying on them, after all, they have a right to know what he's got to rely on.