Mechanically speaking the Mentor character should only take one of the given Guide moves at the start of the game, then through Advancement they gain more. Unfortunately we goosed the advancement rule that helps clear this up, it's in the errata here:
Otherwise a Mentor character has access to all the abilities of a regular one, with the sort of agreed upon fictive conceit that you are acting as a mentor within the game, and you'd be a pretty bad one if you hogged the spotlight, or got in the players way. Try to play with a mind to the character back at base or on the sidelines. The moves, hopefully, try to reflect this style of play with big once-in-a-session supporting acts. I dunno how well I did with those alas 😅. You can of course opt not to use those rules if you're concerned about it at all! Playing through as a more traditional GM/Player relationship works just fine.