K, so you want a character who's going to last and keep being fun to write. This is a trick used in sitcoms and tv dramas for characters who'll just keep on keeping on.
What does the character want?
Exactly what it sounds like. What does your character want right now? It could be a goal, like graduating from Hogwarts, or an object, like a million galleons. BUT it needs to be measurable. We should be able to tell when they've achieved it.
Sitcoms and TV are usually set up so the character doesn't achieve this thing, but RP is different. Once your character achieves their want, you can set them up with a new one. Or they can have a big want, like becoming Minister for Magic, and you can break it down into smaller parts.
What does the character need?
Your character's need is an unconscious desire. It's what they need to be happy, or to grow as a person.
There are two important things to remember about a character's need.
1. It should directly conflict with their want
Michael Corner's want is to make a difference in the world, but his need is for security. The more he tries to make a difference, by speaking out against The Order of the Phoenix, or acting against the government, the less security he has. Those things potentially put his family in danger, and even threaten his marriage since Terry is part of The Order.
2. It should be something that can't be measured
Security can't be measured. It doesn't matter what Michael does, he won't ever feel like he and his family are safe.
What is their strength?
We're not talking about a list of strengths here, we're talking about the trait your character will rely on when everything else has failed.
Strengths don't have to be positive, either. The very first RP character I created, her main strength was bullying. The important thing is that your character's strength is going to help them get their want.
What is their weakness?
A character's strength and weakness could be the same thing. Terry Boot's strength is his intelligence, but his weakness is the way he overthinks everything, which is almost the same thing.
Just like the character's strength is going to help them on their way to their goal, their weakness is going to hinder them.
What is your character's main fear?
We're not talking about their biggest fear, or their darkest fear, we're talking about the most relevant fear. How do you know if a fear is relevant? If your character is blinded to the consequences of their actions when they're confronted with this fear, then it's the most relevant one. You need it to be something that will cause them to lash out without thinking.
What does your character love?
Whether it's a possession, or a person or even themselves, your character loves something. Placing whatever they love under threat places pressure on your character (which is what we want, of course).
How do they see the world?
Are they an optimist or a pessimist? Be careful when you're answering this question, it can be tricky. A bitter cynic can become that way because they're a jaded optimist.
Be careful when creating characters who are pessimists. They are difficult to create realistic motivations for, since they're inclined to apathy. Whereas optimistic characters are more self motivated.
You can also think about how you want the world to see them, but in RP we don't really have control over that.