There's a reason why Mattis was recruited into the Resistance.
Possibly his parents were rebels during the Galactic Civil War, and the Resistance believes that it would be a good idea for whatever reason to recruit him, maybe so he can be a great pilot. But for now, he stays in the lower deck.- It turns out that he's Force-sensitive, which might've played a part in his recruitment.
Jo Jerjerrod is related to the late Moff Jerjerrod and is actually working for the First Order.
He could have a Heel–Face Turn because of the Power of Friendship, is killed by the First Order for becoming useless, or gets arrested/killed by his friends.- He's betrays the group in the second book and joins the First Order, so there's definitely something going on. He doesn't seem to have made this choice out of joy, though.
Alternatively, while Jo may be related to Moff Jerjerrod, Jo is suffering from Sins of the Father (or grandfather or whatever) and is actually trying to make up for his ancestor's actions., hence, why he seems quiet; he's actually timid.
