Low ego about their ideas and what they build. I.e they can be wrong, thrash their own code without any issues, and work on other people idea.

But if they can't strongly advocate for an idea or against an idea they don't like and just give up, you don't get their full 'utility'. Which, tbh, is not a big issue with juniors with not a lot of experience, but still.

'low ego', but not too low. You want passionate debates.