A big part of creating an oc for a series often involves the interacting of pre-existing characters and the one that belongs to you. Which is great! Sometimes the interactions between them are good, and sometimes they're absolutely awful.

I'll be discussing the main types of relations between characters; romantic, familial and platonic. All of these are important but most people tend to forget that there are problems with the later one too.

Romantic relationships
We've all seen in some way, shape or form. Oc x canon. If you've been in any fanbase ever, from supernatural to my hero academia, you've probably stumbled into someone shipping their character with a preexisting one.

And that is 100% okay. If you don't follow me and haven't paid much attention then you probably won't know that I completely support oc x canon. Sometimes it's incredible watching your favourite canon character get along with this well-written character that's come from the brain of some brilliant person across a screen. A lot of people don't support oc x canon because it's seen as a Marysue trait, and to an extent, I agree.

Though I do have to specify, having one or two traits of a Marysue, does not mean your character is a Marysue. I hate seeing people attack characters because they're pretty or popular when there are real life people like that. Marysues are bad because they're unrealistically perfect and that's the issue.

Some oc x canon is done so well you can see it being in the series itself, but sadly that's incredibly rare.

A lot of oc x canon is done by newer writers, which is why it gets such a cringey and horrible stigma, so here are some things to definitely avoid if it's your first time writing something like this.

-Don't make it so that the characters have previously interacted. It's often seen as an easy way out, especially when it comes to colder characters. It's fine that characters have a past together, but please show it, don't just reference it and then have them make out the first time we even see them together. Doing this gives the reader nothing to route for and it can be frustrating to watch.

-Don't make every single character fall head over heels for your character. I, like a lot of people, tend to avoid stories that are just shipping their character with everyone. For example, how many "oc x mha" have you seen? I recommend shipping your oc with two characters at most, and only if you think that you're up for tackling a live triangle. Having every guy, gal, nonbinary pal fall at your character's feet is incredibly unrealistic and that's when your character is going into Marysue territory.

-Don't make your character's relationship the focus of their personality. Reading a story that solely focuses on a character's interactions with another character gives the impression that they were designed purely to be shipped with a canon character, which comes across as self-inserty (I don't have anything against self-inserts but purely from a writing point of view, it's just a really bad idea). If you want people to love your character, don't try and push them with someone. A lot of incredibly beloved characters aren't flat out forced into a relationship with someone, mainly hinted about feelings, for example, Levi from attack on titan (note: I don't ship anything from aot but I do believe there would've been romance between him and Petra had she survived).

-Be careful shipping with cold/mean/uninterested characters. This problem with people loving characters like Levi, or Bakugou or even Dazai from bungou stray dogs is that they're shipped with a lot of characters and ocs. But they're super hard to make work in a relationship. You either need someone that's attractive, fiery and actively goes against them in a flirty manor to create some kind of sexual tension, or a complete leach that just sucks up to them. If you can find another way that works, then please use it. But the main thing here is that they probably won't work with shy characters because your character would have to make all the moves. They ain't nice people, they aren't going to chase after someone, your character has to be the pursuer.

-Love at first sight is boring and rarely works. Unless you're shipping your oc with the most hopelessly romantic cinnamon role, this isn't going to work. This kind of relationship lacks the tension and the flirting and all the good stuff before the relationship starts because you already know that the canon character is head over heels. This will only work if your character falls for the canon and the canon character slowly begins to open up and develop feelings, but you don't see that because normally one of them confess in barely any time, then bam, they're making babies. This stuff works for one-shots, not plots.

I think those are all the main issues I see in oc x canon, but another set of issues arise with familial relationships.

Familial Relationships
You might find this odd, because whilst I support oc x canon when it's done well, I very rarely support an oc being related to a canon character. It's often something I see as an easy way out and not having to flesh out the character's family. Most of these characters are related to main characters and are trained and super strong because of who they're related to, and I think that's why I have such an issue with it.

Though I don't have nearly as much of an issue with adoption stories, maybe it's because they have more freedom with creativity. But, I prefer them when we see the character's first interactions with each other. Watching character's entire relationships unfold to make it so much better and that's why them being related causes the impact to be lost because you don't see the entire path if development.

I've never written a character like this and I often avoid characters that are related to canon (not including second gens), so there's honestly not much I can say here without it coming across as inaccurate, and I'm not up for feeding people false information.

In general, though, I do recommend avoiding this because it limits the possibilities that the character could have.

Platonic relationships
If you're throwing your character into an existing universe, you might as well make them friends with some of this cast. The keyword here is some. Having your character be friends with everyone is unrealistic.

Another thing that's unrealistic is that a lot of people write the characters friends having a crush on them. Can I please read a story where we have a friendship where the characters are just that, friends?

This type of relationship is done well overall, so there's not really much I can say here. Just try and make characters that have an interesting dynamic friends, because that way you route for their friendship and you want them to stay friends.

Overall, there's a lot of different types of relationships and obviously, I have a lot of varied opinions on them, but I hope that what I've said is at least somewhat helpful.

*These tips are things I've thought of after spending nearly five years on this site and been reading nearly my whole life. If you disagree with anything I've said, that is completely fine and you obviously don't have to follow anything I've said*