Well, a lot of rps encourage balanced characters and a lot of players strive to have them, because they're the most fun to rp with! I balance my characters by first coming up with a base persona, with broad adjectives and an equal amount of positive and negative. To begin with, i'd start with something like:
(GOOD) (BAD)
~Fast ~A bit rude
~funny ~jumpy/scared easily
~smart ~doesn't like kids
Notice how, not only is this character balanced, but also that even the bad traits are fun to rp with! You can also always throw in a disorder or something too to really spice up your character! As you rp more with this character, you will see more opportunities for bits of their persona to emerge that weren't there before, such as fears, favorite foods, crushes, and their likes/dislikes.
As for a backstory, these normally are formed later on in character development when you can really work out all the kinks and figure out something that fits the character. Doing it the other way around works, too, i have just never done that before.
For example, I had one of my characters die and so I made a regen char of a puppy, who is now a little over a year old. She was originally pack-born, nothing special, but later I managed to create an intricate and interesting backstory that connected several parts of the plot and other characters and boosted character development even further!
It just takes time to get to a fully balanced and developed character, just keep rping with them and adding new things into the mix (but be sure traits don't contradict each other
)