I've been planning to write this for some time, but I'm a lazy prick, so I just got to it. Also I've been busy, but mostly I'm a lazy prick. But anyways, to me this is very important because when you create a character, everyone instantly loves/likes/hates that character. Which is actually very stupid unless in canon, the character who loves/hates your character has a reason. Like your character is a cat neko, and the character in question is allergic to cats, the character in question would probably dislike your character. No matter how you try to get around. But there is another thing I would like to point out, interactions can change dramatically within different fandoms. So each paragraph will be about a different fandom.
First up, Creepypasta. Ah yes, the fandom that believes a sociopath would fall in love with them. Yes, I know that is mean to say. But it's true and highly illogical, due to they don't fucking exist unless you have physical evidence. Anyways, in CreepyPasta, your OC is very unlikely to meet any CreepyPasta ever unless they are literally hunting them down. Which is weird, don't do that. Also, don't have CreepyPasta a know and popular thing in the story, it's highly illogical and let's just say I would feel bad for Jeff if his fangirls became CreepyPastas. But when making interactions, put thought into it. Personality, appearance, and powers can dramatically change how they would interact. Think hard and long about it, just use that pinkish grey thing between your ears.
Now it's Hetalia. One sentence; just look up the country your OC is based off about and find about its relations with other countries. Its not that hard.
Next up, Black Butler. Now, you do as you please. Just don't make every character love you immediately. Again, use your brain. That is, if you have one. Wait...
Second to last, Homestuck. Now this is one is complicated, just like the plot of Homestuck. How do they know the characters, were they friends/moirails before hand? Or do they never meet the canons and its a while new group of weirdos? If it's the second answer, then how did they meet the others and do they get along? But keep in mind that the story starts out with all of them roughly being 13, so none of them would have lovers before Sburb/Sgrub.
Lastly, The Evil Within. Its really the same thing I said as I did in the whole part for this fandom. Ruvik will probably never love your character, because he is a heart-less bas-turd. So if your character is a cop or a detective, they have a chance of already knowing Sebastian, JoJo, or Kidman. But if they're a patient at the hospital, then they are more likely knowing Leslie or Dr. Jimenez. Just use logic and you should be fine.
So in short; use logic, your brain, and every detail on your character to make the interactions.