Permission was given by the OP for me to reblog this separate from the post it was a response to - it’s just too good to not share it around on its own.
Guide for pairing an OC with Yagami Light:
For women, mature and physically attractive; consistent in your style so that you project being secure in yourself, but with enough insecurities for Light to exploit and control. Stay on his good side by being willing to let him have complete command, and don’t embarrass him (using the strictures of Japanese culture as your guide). Be a meek and perfect maiden. Light is, at his most balanced, a sociopath, with a skewed ability to become emotionally attached if he’s capable at all, and even his creators have said he’s incapable of loving a woman. So appearances are all-important! To catch his eye, you’ll need to have either status or ambition with the looks to back it up (unfulfilled ambition would be repulsive to him), or be of use to him some other way. He has a long, long way to go before actually being able to form a bond of affection to start with, so be patient and accepting when he puts up his walls. If you see those, downplay however you set him off and distract him, or he’ll start to think you’re more effort than you’re worth. If he’s looking for anything, it’s an accessory to hang off his arm so he’ll appear as what he believes others perceive as “normal”, and as an ambitious person himself, that accessory’s reason for being in his presence is going to be tied to what he can get out of it to advance himself. He admits in canon that he dates meaninglessly - in his innocence he will try to maintain the image of a Good Boy who will not play a woman false, but give him a little power and all bets are off, so be careful of your own emotions if you want to keep his company.
For all other genders or lack thereof, there is a distinct hurdle at first, because Japanese culture is pretty heteronormative, and Light’s all-important projected image is dictated by his culture. However, you’ve also got a leg up in that Light sees women as easy for him to manipulate, and therefore unable to be his equal. And what Light desires more than anything else is an equal. He’s bored; absolutely everyone in the world seems beneath him. Prove him wrong by existing. That alone will challenge him and get his attention once you’re within range. Then, if you want to be on his good side, be as much like him as possible, projecting the same societal virtues. He deeply appreciates meticulous care and, above all else, intelligence. But again, to stay on his good side, be willing to let him take the lead. Nothing must challenge his essential dominance, or else you’ll be perceived as a problem, or worse, threat.
If you’re not bound by desiring entirely positive interaction and are willing to take the risk, and are able to challenge his intellect, then you’ll really have his attention. This can actually become a problem because he will become interested in you to the exclusion of all else, and will bend his own rules in order to try to figure you out. Your best bet is to remain as mysterious as is feasible for as long as possible - that’ll keep his interest, and that’s what you want, especially if he decides that you’re a threat. This is the only type of interaction with him in which you will begin to see him rather than his facade, though, so if he’s what you want, it’s worth it. He will detest you, but he will hide that as best he can in order to stay close to you in the hopes of solving you - but nothing compares to that entertainment for him, so you will become the most important person to him in all the world, and that could be the start of his long path to affection. Plus, you get to be yourself regardless of his tastes - but it only works if you really are at his same level of intelligence.
In all cases, getting clingy or emotional will be a true turn-off for Light; he values intellect and views emotion as a dangerous opposing force to such. He’s also unlikely to put much value in physical relations - beyond appearances, he actually doesn’t have much interest in intimacy. It’s more trouble than it’s worth when it’d be just another form of manipulation and he can do just fine without it. He might realize the value in it with someone with whom he could be emotionally attached, but… that puts us back to square one.
He’ll almost never be an attentive boyfriend in anything more than a surface sense - again, that would take emotional attachment - but he is a master at going through the motions. He won’t forget your birthday or your favorite food/color/season or fail to impress your parents, and he’ll be an absolute pleasure to be around and a really impressive catch. Just let him be the one to break up with you, even if it means throwing a fit about catching him two-timing you - if you call him on being all about appearances, or act outside the role he wants you to fill, you might get classified as a threat.
If all you want is to be in Light’s company and never get under that mask he wears, support Kira from an unemotional standpoint for the good of society and be ready to intelligently argue in Kira’s favor at the drop of a news anchor This way you’ll stay on his good side, will exhibit enough intelligence to be at his side to begin with, and you’re likely to survive the experience. Just don’t be too enthusiastic; he wants someone who will speak his truth and understand it, not proseletyze. It’s just common sense to support Kira, after all, so you only need to talk about it when somebody says something stupid, like that Kira’s just a murderer. If you display no knowledge or opinion on Kira, you’re less likely to hold Light’s attention, as not being aware of or carefully considering current events reflects poorly on your intelligence and ability to be useful with your status or ability to fulfill your ambition.
If you want to have a chance of seeing the real Yagami Light and challenge his intellect, just state the blatantly obvious (that he is Kira) and be ready to back it up and debate at length. The mental stimulation of refuting evidence and debating will keep him entertained and, while classifying you as a threat, mark it as an enjoyable experience for him, so you’ll keep his attention. Just don’t let him know your name.
Light is bored; he can manipulate pretty much anyone, wants challenges he can’t find, and there is nothing new under the sun. Having known him for a long time only makes you a known quantity or a solved mystery. It’s highly unlikely that he’ll secretly carry a torch for anyone he may have known as a child - after all, they may have insights into him from when he was less accomplished with his facade, and that’s just too dangerous to have around. It’s even very possible that his appearance and intelligence got him disliked in his youth, hence his learning to cultivate such a perfect, ingratiating facade. Given how rapidly humans mature in their teens and early twenties, it would be unlikely for a relationship formed on mutual interests/tastes in youth to have any meaning thereafter for someone as guarded as Light.
He’s always looking for a challenge, so Light is going to be more attracted to someone new - while it won’t be anything unexpected, figuring out a new person is at least mild entertainment for a while. College classmates or co-workers *cough* with whom he has repeated interactions will be the pool he chooses from for dating. Don’t try anything cute like spilling your coffee on him “by accident” and demanding he send you his dry-cleaning bill so you get his contact info, or vice-versa; he’s a guarded and suspicious person and you meeting with some unexpected but completely natural accident is a far more likely happenstance than him starting a relationship with someone he meets that way.
That said, he won’t rush a relationship either; he wants to appear to have high standards and be looking for some true romantic compatability, not gain a reputation for being a fickle womanizer (that only enhances his prestige with other men, and he’s too savvy to entirely discount the power of womens’ opinions of him as well). The relationship will play out according to his terms, and in the end, he wants breaking up to be decently respectable, most of the time. Throwing a fit about him two-timing you will get a graciously worded apology and belated realization that he does not love you, but you will leave the relationship feeling strongly complimented and appreciated regardless of your personal incompatibility.
If you don’t, well, he does know your name.
Don’t expect the relationship to last indefinitely - he’ll get bored and look for the next semblance of a challenge as soon as he’s solved you. The only way to keep his attention and genuinely win his affection is to be able to remain a dangerous unsolved puzzle long enough for affecton to actually take hold, and that’s very nearly impossible. It’s dangerous, but remaining unsolved while being abhorrent to Light and irritating him will both keep his attention and connect you to his actual emotions. An irritating challenge is better than no challenge or a disappointing one, so he will come to view that irritation with affection. So if you want a true relationship with Light that will have real meaning for him, you might not survive long enough to see the flowering of your efforts.
So, in closing, pairing an OC with Yagami Light: it’ll probably mean a lot more to the OC than it does to him. Short of personality alterations he’s a pretty irredeemable monster with just a few tiny outside chances of fulfilling but unhealthy relationships, and dangerously bored and amoral/egotistical enough to turn out terribly bastardly even without the Death Note (as exhibited in the first episode). I appreciate the work of anyone who puts their OC through that! :D