One thing I've noticed while rping here is that it's hard to make a good character. Usually, you only have a few weeks or so before your rp dies and you barely have a chance to develop your character.
So without the time to develop, you have to put your best foot forward and hope for the best. However, people don't usually tell you if you're doing a good job or if they like your character. So you never learn what works and what doesn't. That's what this is for.
Basically, try to think of a character you liked in an rp, say who used them, and why you liked them.
For example, I like Hyde's character Taiga from Pokemon Gijinka: The Fallen God. I like her hot-blooded-ness, and how it can get her in trouble. She's also tsundere in a way, and while I usually don't like those characters, Hyde plays her in such a way that she's likeable.