(Few time after Tsunade turns into a Hokage) Kakashi sensei was in a solo mission and he didn't had time to give some lessons to Team 7. You're one of the best shinobis in Konoha, so Tsunade, selected you to be Team 7 mentor while Kakashi was on the mission. You accepted the offer and few hours later you go meet Team 7.

Naruto: "Who are you?" He asked while he stares you with confusion and curiousity

Sasuke: "This might be our new mentor."

Sakura: "Our mentor?" She glares at you