Oh my Gods y'all, Malec would totally be that couple...
The ones who go all moony- eyed when they're talking about each other; while it's adorable and sweet, it can often get in the way of doing things: Alec was walking down the hallways of the institute, Jace walking briskly by his side, when Jace made the mistake of asking how magnus was doing. By the time they'd stepped out into the grounds of the institute, Jace had gained enough knowledge to write Magnus's biography- he knew about his favourite flower, how adorable he looked with Max in his arms, and how Rafe had come close to stabbing him when they were sparring.
Jace got fed up enough that he walked away halfway through a battle with a couple of demons (it wasn't anything Alec couldn't handle), and Alec, too lovestruck to pay much attention, didn't even notice his absence until he started his walk back to the institute. Jace had stolen his stele too. Ass.
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They always have to be touching each other - after they had settled into their roles of most famous celebrities of the shadow world, they were comfortable enough to allow PDA in front of, well, pretty much everyone.
Now, while this sounds cute and intimate, it can get rather awkward and irritating for... well, literally everyone else ...
Once, they had stayed over at some or the other institute for a night. The owners happened to be homophobic. In the morning, when they were all in the dining area for breakfast, Alec, exhausted from the mission, made it a point to sit on Magnus's lap. Magnus put his arms around Alec's waist, giving him a small kiss on his forehead. The wizened old woman, one of the occupants of the institute, gasped audibly and pointed with a wrinkled finger across the table at them. Unbothered, they ate breakfast in silence. They were, frankly, too tired to care. The other occupants were also slightly slack-jawed at the public display, but after the second or third day, they all got used to it. The old woman still nearly went into cardiac arrest when she stumbled upon them making out in a hallway, though.