This is a long and tricky achievement with little hand holding along the way. What you have to do is either treat your first companion, Innocence, bad or good through conversations or responses to events unfolding. To not make this too long a complicated, know that being a bad guy in Mars War Logs will put you at a serious disadvantage. The following is the good-guy path to unlocking Mentor.
You will usually get three responses to choose from, and for the most part. A good, a bad, and a neutral response. Seldom do they mix it up a little bit and give you only two responses, or two neutral. Note that this will contain some spoilers.Chapter 1Opening sequence, right after the intro
Good: I couldn’t just stand there and do nothing
Good: It’s nothing. Let’s get out of here.After beating up Fatso
Good: Let’s go to the Mess Hall. We need to talk.Mess hall first conversation
Good: Yeah, but it’s not going to be easy.
Good: Want me to go with you to recruitment?Optional conversation in Camp 19
(Talk with Innocence, Y, after the Mess scene)
“Where do you come from?” Good: Nothing exeptional.
“What battles did you fight in?” Good: I kept my head down.After you come back up from the Mole Caves and go to the Mess again
Good: That’ll do. You’re coming with me now.When asking Jey about the picture during the Another Face side quest.
Good: Don’t worry.I’ll be there.Chapter 2Talking with Innocence after opening scene.
“You know Shadowlair Well?” Good: I’ll have to show you the slums.After you reach Innocence's parents shop for the scene, talk to him again.
“You said your parents were in the militia?” Good: A noble intention…Talk to Innocence again right after the above.
“You holding up?” Good: “You know you can count on me.”
There are plenty other conversations, but they don't lead to you making a response. After the above, you have influenced him as much as you can do, and the achievement will unlock when Chapter 2 ends by you picking a side.
From my site The Role Playing Grind: http://ellusion-no.blogspot.no/2013/08/gamehelp-mars-war-log...