Here I would like to share my general thoughts about how could it be on surface level, no deeper
At first, you wouldn't even notice his presence. Zack isn't the type to spam you with messages, leave hundreds of notes, or give you any hint that he's around. He prefers to watch from a distance.
Soon enough, you'd start noticing strange things. For example, you always make it home safely, even though you usually end up in some kind of minor trouble. If someone treats you badly, there's a chance that a few days later they'll have a very unpleasant conversation with an extremely tall guy wrapped in bandages, end up with a few bruises, or simply disappear altogether, who knows what kind of mood your "bloody angel" will have.
Zack would deny being interested in you until the very end.
"I'm not stalking you, idiot! I just happened to be nearby."
Even though his version of "happened to be nearby" has been going on for three days straight.
He knows your daily routine perfectly. The routes you take, the places where you like to sit alone, the time you usually come home.
If he notices you're upset, he'll get irritated. Not because he knows how to comfort people, but because he can't understand who dared to ruin your mood.
The funniest part is that Zack genuinely thinks his behavior is completely normal. In his mind, he's simply making sure nobody hurts you, because you're obviously incapable of protecting yourself and need someone watching over you. He would never admit how much time he spends observing you. If you suddenly change your usual route, he'll become noticeably anxious.
One day, you might accidentally catch his gaze from the window of a nearby building and then he'll simply walk away without a word, as if you're the one who just did something embarrassing.
Of course, he won't be able to hide his stalking forever. When you finally ask him directly:
"Are you stalking me?"
Zack will freeze for about ten seconds before muttering something like:
"So what if I am?"
There won't be any fancy excuses. He'll honestly admit that he just wanted to make sure you were okay, and he'll be genuinely surprised if you get angry about it.
If you do get angry, he won't show up for several days after the argument. But eventually he'll come back anyway, just keeping a little more distance and trying not to get caught watching you.
And if you don't get angry, and his weird feelings happen to be mutual... then both of you probably need therapy.
Zack has absolutely no idea how to court someone. Instead of flowers, you might find a bag of your favorite sweets or something you've wanted for a long time sitting on your doorstep (stolen, most likely) and honestly, you'll be lucky if you don't find somebody's organs there instead.
He's extremely jealous, although he'll try to act like he doesn't care. His jealousy usually manifests as aggression toward both the object of his jealousy and whatever caused it. Zack doesn't really understand the feeling itself, which only makes it annoy him even more.
He loves physical affection, but rarely initiates it himself. Looking at him, you'd never guess he enjoys that kind of thing. However, if you hug him first, he'll instantly go quiet and become unusually calm.
Sometimes he'll simply sit beside you in complete silence. For Zack, that's already a huge display of trust.
The most important thought in his mind would be:
"She's/he's/they're safe. That's all that matters."
And if someone ever tried to hurt you, Zack would place himself between you without hesitation, even if it meant getting hurt himself. Of course, his goal wouldn't exactly be to protect you, his real intention would be to eliminate the threat permanently.
