Roxas thinks Isa is a serious, decent guy, who has a lot of his own team's notable traits. Thanks to Emerald, Roxas is going to react instinctively well to stoicism and open communication, which are both traits Isa has exhibited. So his first impression of Isa is a good one. That, and it's not like one of his favorite people in the world didn't cause an apocalypse (hi, Knife!) So Roxas's attitudes toward trust in this case are: a) you are who you are now, b) as long as you're capable of remorse/change if you learn you were someone the current you doesn't like, and c) if I mistrust everyone on sight, I'll go insane. Additionally, he's Axel's friend, and while Roxas knows his friendship with Axel wasn't all peaches and cream back home, he also values it, which means he's automatically going to regard Axel's other friends positively. He tends to take a 'friends of my friends are my (potential) friends' attitude, until and unless proven otherwise.

How are Saix and Axel doing right now?