WOULD THEY CHEAT:
LIGHT:
Yes he would, though when he was innocent Light he seemed very against doing things like that. I think he liked to see himself as someone who would never do that to anyone, but once it came down to him achieving his goals he’d pretty much do anything to anyone to succeed.
If his partner was wary due to being cheated on before, I don’t think that would stop him from doing it to them again at all. He’d probably be very good at assuring them that he would never do it, though. He’d know that his partner was insecure about it and be good at telling them exactly what they needed to hear to trust him. He’d also probably secretly find it kind of unattractive that they were insecure.
L:
I don’t think he’s the most morally upstanding person on the planet by any means, but he also doesn’t seem like someone who would have a wandering eye when romantically attached. I think it takes a lot for him to want to be with anyone in the first place, so if he’s in a relationship he’s very into them already and probably not looking elsewhere for satisfaction.
I don’t think he’d be extremely sensitive about an insecure or wary partner because he’s fairly impatient with people and not at all insecure himself. He also probably wouldn’t want to do a ton of coddling of their feelings. He would just think it was obvious that he wasn’t going to do the same thing to them and that they should understand that and get over their fears already.
NEAR:
No, I don’t think he would. He seems to be very straight-forward and loyal and honest in his relationships.
He’d probably be pretty good with an insecure or wary partner, as he’s trustworthy and good at seeing things from other’s POVs when he likes them. He wouldn’t give them any reason to doubt his loyalty and he’d be patient with their insecurities, too.
MELLO:
I think that he would. He’s very much out for himself as an adult and doesn’t spend a lot of time caring about other people’s feelings. He’s probably loyal to a select few trustworthy friends but couldn’t care less how he affects everyone else. I think he’s cynical and expects other people to screw him over and would probably much rather be the one doing the screwing over first.
I can’t see him being that great with an insecure s/o, either. He’d find it annoying and troublesome, as he’s got plenty of his own problems and ambitions to worry about without having to cater to somebody else’s emotional needs.
MATT:
It’s hard to say because we know so little about him, but I’m gonna go ahead and say no, Matt wouldn’t cheat. He might check out other people regularly and flirt a little outside of his relationship if he’s drunk, but he’d never do anything super untrustworthy if he was in a relationship for real.
I think he’d try to be good with an insecure s/o, though possibly would end up just cracking awkward jokes that make things worse instead of being incredibly sensitive to their concerns.
BEYOND:
I like to think that B is actually the most fiercely loyal person in the world when it’s somebody that he likes, and that he only has eyes for that one person and nobody else, to an almost obsessive degree. So no, even though he’s certainly no paragon of virtue I don’t think he would ever cheat in a relationship.
I think he’d kind of like it if they were insecure because that means they’d be more likely to cling to him back and not have wandering eyes themselves.