1. Consider the personalities of both characters carefully. You’ll want to truly understand both characters in order to foresee the directions the relationship may take. Your initial hypothesis may be incorrect; however, you will have considered each direction carefully and decided which are the most plausible. There are an infinite number of ways the two characters’ personalities might work with each other, but here are just two paths the relationship could follow:
Assume your character is a very strong, opinionated individual who sticks by his beliefs, right or wrong. Your character holds a strong opinion on an issue dear to him, and the angry character holds the opposite opinion. If a heated argument over this particular issue is to arise, and both characters are equally as steadfast on their position, your character is likely to develop a strong hatred for the other character.
Now your character is a compassionate, kindhearted individual who tries her hardest to find the good in everyone. The angry character might be misunderstood, possibly with a dark past, and your character makes it her mission to uncover the secrets and get through the anger. In this way, your character would be likely to develop a love for the angry character.
2. Basically, the rest of the relationship is all up to the way your character reacts to the angry character. Now that you’ve considered both characters carefully, you’ll be able to better understand the anger of the angry character, and better write each reaction of your own character in your paras. What if the angry character yells at your character for something he’s done? Does your character get angry and yell back, or try to calm him down? Does your character try reasoning with him, or blame someone else? What your own character does will define the direction the relationship takes.
One of the first characters I ever played had a relationship with a somewhat angry character. In fact, here is the very moment in which my roleplay partner and I first knew that they were going to fall in love. Adrian is beyond angry, angry at something he believes another person has done to him, and Adryn’s reaction is to simply wait as he yells his frustration out. Then, she says four quiet words, and in the very next paragraph you can see the calming effect those words have on Adrian. This is one way that a character’s reaction to anger can turn into a more powerful relationship.
Two of my friends roleplayed together and their characters are the perfect examples of a great and interesting relationship built on hatred. Victoria is the type of character who hates the world, and has a bone to pick with nearly everyone. Billie has an extremely short fuse, and Victoria’s negative attitude really sets Billie off. Here, Billie has just returned from a weekend trip to Chicago alone with her History teacher, and Victoria begins taunting her about it. Billie reacts in extreme anger, and here it gets to a point where they are using their superpowers to nearly tear each other apart.
3. There will be some characters who will simply be completely closed off and will act like they don’t want to talk to anyone. This can be frustrating, as it gives you little to no material to work with. My only advice in a situation like this is that if you really want a relationship with that character, keep at it. Don’t give up. Try to find something your characters have in common with one another, or something they have to argue about; both of these things will give you fresh material to write about!
4. Make friends with the angry character’s roleplayer. Talk to that person and come up with headcanons about your characters. Plot storylines! Discuss the direction you want your characters’ relationship to take! This is the best way to ensure that you will have lots of ideas to work with, and you won’t have a reason to give up.
5. Don’t be afraid to give up if absolutely nothing has worked. It is true that some of the best and most interesting relationships can come from polar opposites, but if the relationship is going nowhere, there are other characters to roleplay with. Promise to try the relationship again later, and then forget about it for now. Maybe later on, when you have a fresh perspective and both characters have developed more as individuals, the relationship will fall into place much more easily (who knows, they might find more things they have in common during this time!).
I hope this was enough information for you, although I feel like it may be a little too general. Let me know if you want me to elaborate on something, or add something, or if you ever do have a specific situation you want me to offer advice for!