So I said I was going to do this and then I kept falling asleep trying to write it into something coherent but I am going to try again anyways! And it will make no sense so you can ask questions if you are confused. And if I contradict myself somewhere feel free to slap me.
Basically Elliot is the epitome of everything good and wonderful and light. HE IS LEO’S DISNEY PRINCESS, OKAY. Prince, Princess, whatever they’re both shiny and bright. At the House of Fianna Leo had nothing but books and playing the piano, and then Elliot came along and there was something to look forward to. The Nightrays are bad news, Leo knows, but Elliot is just…dumb and noble and obviously not like the rest of his family at all. Elliot gets to essay on how he is the only child that did not get screwed up, I am not touching that one. The weird thing about Elliot is that he is just pure no matter what. Everything he does is for some stupid noble good reason. God, even when he’s going around killing his family members Elliot is doing it because he’s trying to protect Leo. (Well, Humpty Dumpty was protecting Leo. But I am sentimental and terrible and like to think that the only reason Elliot couldn’t fight back against Humpty Dumpty in those instances was because he wanted to protect Leo too-just in a less extreme way.) So even his terrible acts are because of the good inside of him! And he didn’t know that he had done those things and is constantly fighting against Humpty Dumpty unconsciously and not giving in to the insanity. So even though he has that “darkness” inside of him, it’s essentially not his fault and he stays pure through all of it.
Humpty Dumpty is Leo’s fault, though. So the only reason Elliot has any kind of darkness in him at all is because of Leo. And it’s something he can never forgive himself for, no matter what Elliot says. Because they’re opposites Leo knows that if Elliot is light, that would make him dark. He’s a little off-kilter and has weird voices whispering to him and sees golden lights everywhere and knows that he’s weird and tries to reject it. But Elliot, gosh, Elliot is fascinated by it and finds the beauty in it even when Leo can’t. It balances out all of the negative feelings that Leo has and the self-loathing that comes with knowing you ruined your only friend’s life.
There’s also some kind of recognition that Leo is not bad for Elliot. He is Elliot’s friend! He’s someone who Elliot can experience positive interactions with, because he doesn’t get that from people back home. They both rely on each other because they don't have anyone else to turn to in canon; it's not until Oz shows up that they even have other friends besides each other. Which kind of brings up the whole issue of how much Leo is really needed in camp, since Elliot has friends and no one is ostracizing him for being a Nightray and Leo’s role is growing more obsolete.
Except that it isn’t! (Wow, I am good at sense-making, okay.) Because Elliot still needs Leo just as much as Leo needs Elliot. They can function without each other, but for Elliot it’s a little easier because he at least knows how to interact with people. Leo just automatically shuns people because he doesn’t want to deal with them-but now that he’s Elliot’s servant he has an obligation to interact with others. So even while Leo helps Elliot make sensible decisions, Elliot helps Leo to not hide as much as he used to. It’s…okay yeah it’s a codependent relationship because they help pull each other into balance and without that balance Elliot does dumb things and Leo goes crazy.
And this is not to say that Elliot is perfect-he’s not, and Leo knows that. He makes mistakes and yells and does stupid things and resorts to violence way too often. But in spite of all of that he is still noble and wants to help people, even if he goes about it in his own special way. Even amnesia’d Leo was just…baffled by Elliot and his stubborn Elliotness. Why do you care so much, stop caring it’s weird. And the whole time he just had this overwhelming feeling that he didn’t deserve what was being thrown at him, because he didn’t remember and he wanted to know why Elliot thought he deserved such a thing. He knows he's messed-up and weird and there's a reason he's hiding behind his glasses and his hair and everything he does, but Elliot just pulls him out of all of that anyways.
…And then I ran out of steam and forgot where I was going with this. tl;dr this is a completely healthy not co-dependent relationship and they are not gay for each other at all. REALLY.