Jace: he'll look you in the eyes and give you a smile, before he hugs you. That's enough. He isn't the hugging type, but when he does wrap his arms around your waist, you know he's trying to tell you he loves you.

Alec: he'll say something along the lines of: "I trust you," which always makes you realise that the oldest Lightwood doesn't trust that much people. So when he states that, he is trying to let you know how much you mean to him.

Magnus: maybe it is the weird nicknames he calls you, or how he tells you he hasn't let anyone in for the past century. Perhaps it is when he borrows you his jacket once you're cold or kisses your forehead gently before saying: "this never grows old."

Simon: he'll hold your hand, softly and genuine, before he kisses you on your cheek, his own cheeks flashing red, while he says reassuring you and anyone close enough to hear: "you're amazing." When he's in his silly mood, he'll start singing "NO ONE DRINKS LIKE Y/N NO ONE SINGS LIKE Y/N!" Leaving the Maybe weird or awkward things out.

Raphael: he shows you he loves you by not wearing any silver rings or necklaces, and anything else triggering for wolves. You'll do the same, not wearing anything with a cross (if you even do that) and not bringing a wooden "stick". When things get really bad, like a fight or argument gone wrong, he'll stand in front of you and say: "I'll protect you. Trust me, I won't let anything happen to you."

Sebastian: often, he'll just tell you how he feels. But once every while he'll do so in romantic ways, cooking dinner, buying you gifts. When he feels like doing none of the above, he'll hold you tight, kiss you and then say: "you mean the world to me."