Mentor, who, how, when?

Hello, my question is:
-who is mentor?
-how to get it/him and what it/him gives
-when can i get it (lvl)

thank you for replies

-Anyone lv86+
-By having a lv86+ invite you. Buffs equal to PP buffs + Prophecy (no dance/song effects) as well as +200% xp buff while Mentor is online.
-lv40+

-Anyone lv85+
-By having a lv85+ invite you. Buffs equal to Iss buffs except sonatas (PP doesn't have clarity VR Emp etc) as well as +200% xp buff while Mentor is online.
-lv40+

fixed ^