The thing about Adam and Ronan’s relationship is the fighting.
“If it had a social security number, Ronan had fought with it” This is the most general, accurate description imaginable. Ronan Lynch is a fighter, because he’s a contrarian, because he’s full of anger, because he doesn’t know what else to be. It is, arguably, his primary character trait - that along with how fiercely he loves (which may or may not go hand in hand). Ronan fights with every character except Matthew and Aurora - both dreams. He snaps at anyone. He is devout and yet irreverent. “The annoying thing about Ronan was always that he was angry when everyone else was calm, and calm when everyone else was angry.” He is a fighter and he’s angry most of the time, and only less so when others are angry (unless he’s especially passionate, ie fighting Declan in the parking lot or Adam at work)
Right before the line “If it had a social security number, Ronan had fought with it” Gansey notes that Adam and Ronan must have fought. The implication being that they fought so regularly, Gansey could pick up on it in two sentences from Adam. The implication being that this is not a big deal. The first interaction we see between Adam and Ronan, they’ve fought such that Adam doesn’t feel welcome at Monmouth (but is still willing to answer Ronan’s phone for some reason, I love them they’re so weird) This also colors all of Adam’s narration of him for his subsequent chapters, and they truthfully still don’t know what to make of each other at this stage of their friendship. But that doesn’t stop Adam from keeping Declan out of Monmouth for Ronan’s sake, and it’s also very clear that they’re comfortable spending time just the two of them. They drive places together, Ronan teaches Adam stick, multiple times Adam is just chilling at Monmouth with just Ronan there. They fight - often - but it never leaves lasting marks on their relationship.
This contrasts with the others’ relationships. Most of the characters fight each other. They’re messy teens with trauma and destiny. They’re all very unique people. It makes sense, and usually, they get over it pretty quickly. But even when the fight ends, it has a lasting emotional ramifications. When Blue and Ronan argue about religion it’s brief but it’s clear that it genuinely hurts Blue’s feelings. When Blue and Gansey fight it’s about disagreement, and it leaves them tense and uncomfortable afterwards. When Adam and Blue fight, it’s always explosive, and is connected to an ongoing arc. When Gansey and Ronan fight, rather than just bicker, they both carry that kind of guilt or whatever you might call it. And Adam and Gansey’s fights go without saying. But Ronan and Adam’s fights never persist in their interactions - when they fight over Ronan’s double getting killed, Adam expects Ronan to just show up at his apartment, maybe apologize, and that will be that. They fight at an ideological level when Adam’s at work - and Ronan says some awful stuff - and then they just move on. It doesn’t linger. In the same mantra where Adam tells himself “Don’ t fight with Gansey. Don’ t fight with Blue. Don’ t fight with Gansey. Don’ t fight with Blue,” - something that he returns to often - he says “There was no point telling himself not to fight with Ronan. They would fight again, because Ronan was still breathing.” That’s a comfortable place in their relationship. It still feels secure. Adam doesn’t have to worry about fighting with Ronan, or bickering or disagreeing with him, because it never makes their relationship feel tenuous the way it does with the others. Ronan fights and Adam fights and so they fight each other and that’s that.
Which brings us to violence. Ronan was raised to be a fighter, taught to box by his father. And he grew up to be angry, something he learned from his father. He’s a violent person, often but not always physically. Adam was raised in violence - he’s used to it and he’s afraid of it; “He had not known to be born afraid, but he’d learned.” They were taught opposite lessons about violence. And while Adam fights very hard against it - “Adam pressed that fist against her bedroom wall. So gently. Though every muscle was knotted, wanting to destroy it.” Ronan embodies it “He’d been angry before, but now he was nothing. Just knuckles and sparks of pain.” And Adam is wholly unconcerned by this, calling him out for punching the wall, unjudgementally telling Gansey that he couldn’t fight Ronan’s demons. Adam is afraid, but he’s not afraid of Ronan being violent, and Ronan never gives him reason to be “the choice was death or hurting Adam”
In a series littered with arguments and altercations Adam and Ronan, two incredibly stubborn, argumentative people who are highly attuned to violence, feel fully comfortable with each other, never feel the need to pull back on their arguments because the fights never change their relationship. They understand each other. They’re good.