The mentor in my campaign was a Netti. She suffered a near-fatal injury during a confrontation with Darth Vader.
By the time the PCs found her, she had rooted in an ancient temple. She's slowly dying, but she wants to impart her teachings to the PCs before she lets go.
She has since developed a close bond with each PC, this allows her to communicate and train them, even when they are not around.
First, thanks for pointing the Neti out to me - I was not familiar with them.
Second, that is a really great way to avoid "Mentor comes in to save day" - she can teach, but the players have to act.