Just looking to start a discussion here, take the following, as always, as just my opinion...
Characters, whether your long-term AV, or just a temporary creation, are easy to create, but some are more successful at their intended purpose than others.
For many folks, their AV is simply a costume. The character has their personalty, likes and dislikes, and so on. But the question of their background history may still turn up, and it's worth thinking (a bit) about it beforehand.
On the other hand, there are those whose characters are very much unlike themselves, in both appearance and personality. And often accent. :)
So how do you design a "good" character?
1: Simple is good. Have an overall concept you can explain in one (brief) sentence. "A scruffy urchin who is really the Prince of Smaleria", "A one-armed tinker that loves potatoes", etc.
The more you have to explain your character, generally, the less interested people will be. If you tell people "He's a former professional bear-baiter, who learned the ways of the ninja, and is currently infused with the power of the Lok-Nar, and is destined to seek out the fabled land of Fon-Du-Lac.", you lose them very quickly.
This is not to say you can't *have* a complex character, but the essence should be able to be conveyed briefly.
2: Batman is better than Superman. The *less* power your character has, generally the better off you will be.
Challenges are key to character development. The more you have to work at something, the more interesting the character becomes, and the more you have to interact with other characters to solve problems.
A powerful character has less call for interaction, and their challenges tend to be...well, less than challenging.
Also, there tend to be "God-Mode" problems, where other people perceive you as power-gaming. Note that even if you *don't* do such a thing, just the image of your character as hugely powerful can color people's perceptions.
3: Quirks and flaws make the Character. Flaws are, invariably, more interesting than advantages.
They add dimension to a character, making them more believable. A taste for dairy products, a fear of the dark, a malfunctioning prosthetic, often-repeated catch phrases (Indeed!), an ever-present pipe, and so on.
On the other hand, over-doing flaws is almost as bad as over-doing power. This is often seen as attention-seeking behavior..."My arm fell off again! Woe is me! Everyone pity me! Come and reassure me!". :P Refer to item #1, above. A single quirk is memorable, a dozen usually means all of them are forgotten.
4: Conflict builds character. We, the typists, should all try to get along. However, our characters shouldn't.
Most of us are, I assume, nice folks, who don't really want to argue with people, or try to shoot them with giant Death Rays. But really, what fun would Babbage be, if everyone's characters just agreed with everyone else?
So, argue, fight, have disagreements with other characters. It adds interest, and life to the city. Just try to keep it all In-Character. :)