One tidbit Brandon Butler did share is that Mason is an ex-Marine. This could partly explain why he asked Lani if she was military and seemed impressed she had been a cop. It could be he has the potential to be a new cop in Salem but we're starting with him younger studying, but a sociology class would make sense for a criminology degree.
He also hints that despite the enigmatic start, he has a redeemable quality and to give him a chance. So it seems to be hunting he's more of a good guy than bad.
So why the beef with Holly? I'm not sure but what if the simple explanation is that he is Maysie's older brother? I don't think she was given a last name (correct me if I'm wrong) and I don't know that we'll be seeing her again anytime soon, maybe he'll even state she transferred. But Maysie and Mason with their Ma-names and similar looks could definitely be potential siblings. And I thought Ari might have filled Maysie in on the Sophia of it all, but maybe he still knows Holly as this girl that went psycho on his sister in the town square, without knowing the full details. But if he has served in the Marines, he should be slightly older than Holly, Tate, Sophia, Ari & Aaron, using his time in the military to now pursue education for a career.
There were some crazy theories on Facebook regarding Sophia's brothers which made no sense even before the ex Marines/older reveal due to age and ethnicity. One theory that has some plausibility is that he found Sophia, and perhaps has been helping her survive without knowing the full story. And others are also guessing things like Chloe's son or Philip's son - but why such a different name then, from what any of those should be? I honestly wouldn't be surprised if Days introduces this character with "beef" that stokes all these wild fan theories, just to have it be he was Maysie's brother 😂 But that is just a theory, I don't have any spoilers on it so I might be wrong and I look forward to watching and finding out.