If Nina is a Four, Matthias is a Nine. That pull these two numbers share, a conflicted romance, is baked in. We can also see his Nine-ness in his skill: big man. He’s no Grisha, no rogue; he’s pure tank.
He’s basically a Hitler youth who grew up to realize the system he was taught as a child was horribly wrong. To go along unquestioning, seeking the camaraderie of the group and avoiding any conflict of dissent, is Nine. To realize the truth, to reexamine his principles, is also Nine. One of the only things that will move a Nine to battle is justice.
He’s written as stodgy and dense — a bit of a Buzz Lightyear. The joke’s often on him. Underneath, though, he’s a hero in a very traditional sense. He’ll never be the protagonist, as is common for a Nine, yet he has those traits. I think the author could’ve leaned in a little stronger on that aspect. His physicality is given its due, but his innate bravery is turned into comic relief a little too often for my comfort.