Eren:
He will loose it almost instantly and would probably kill the person. Mikasa and Armin would have to hold him back. He would beat the person half to death, which ended with Levi scolding him and him having to clean till his arms fell off, but to him it was worth it. He would keep an eye on you and go everywhere you go. Which started to annoy you at some point.
"Eren, stop following me."
"I can do what I want your mine."
Levi:
He would be calm about it and give the person 500 laps. Then he would make the person clean. But if he was in a bad mood already he would probably beat the shit out of him. Then he would keep an eye on you and make sure guys stayed a decent amount away from you.
"Levi stop that, you can't keep every guy 100 feet away from me."
"I can do what I want."
"Levi stop glaring at him!! The poor thing looks like he shit his pants!!!"
Armin:
He would be worried that you would leave him for another guy and start crying till you assured himself that your his and only his and that you wouldn't leave him. Then you would snuggle for the rest of the day, and read to each other.
Jean:
He would embarrass the hell out of that person. He would basically make that persons life a living hell. Then he would snuggle with you and give you kisses more often in public. PDA would increase.
Connie:
He would stand off the the side with arms crossed and a pout. You would come over and rub his head. Then you would kiss it and he'd wrap his arms around you and bury his face in your chest with a smirk while looking at that person who was now glaring at him. But Connie didn't care he was living the best life at the moment.
Mikasa:
She would be calm at first and only glare but as they kept doing it she would knock the person senseless with one blow. She would hold a grudge against that person. And give death glares every time they walked in. Since then that person was scared to even look at you.
"Mikasa, stop glaring. It's okay I'm yours and yours only." You'd say putting your hand on hers making her calm down instantly.
Erwin:
He wouldn't do anything because he knows you his and he knows you wouldn't flirt back. He'd act like it was okay. Buts secretly he'd put that person on squads that are more likely to die during expeditions.