A few things of note about Billy that I figured I ought to write out sometime...
First would be the twins thing--Except for hair color, clothing, powers and mannerisms, Billy and Tommy are apparrently identical. So identical the first thing that people who are familiar with him say when they see the two together is that the two could be twins. We actually even see Captain America specifically mistake Speed for Wiccan.
The resemblance is uncanny, especially when considering that Billy is from New York and Tommy is from...New Jersey, if I remember correctly. When the Vision says that Billy and Tommy are supposedly the twin sons of the Scarlet Witch (who, ironically, was Billy's favorite avenger). ...The father was the original version of the Vision. Speed reacts with understandable scepticism, but Billy whips up a whole theory on how their souls (which had, according to Vision, been absorbed by the demon Mephisto and were dispersed after hes distruction) were actually set free and ended up where they grew up. In writer interviews, they've been saying that it's only a theory, so since it's still unconfirmed as to whether or not this is true...it's entirely possible that this whole messed-up-soul thing is going on. The Marvel site even lists the Kaplans as Billy's "adopted" family, and the Scarlet Witch and her relatives as biological. (Grandson of Magneto?)
Second: I find "Wiccan" as a codename rather ironic, as I've read that Billy is actually supposed to be a Reform Jew. I myself observe no particular religious beliefs, so am extremely ignorant as to various important things that would figure into this aspect of a character. So poke me if I screw something up/forget/completely miss something.
Third--his powers! Billy is able to use his desires to cast spells. At this point, they haven't really defined a particular limit to his power. But canon suggests that certain things come easier to him than others. At this point, he can fly and generate lightning at will, but most things beyond that requires him to focus his power to achieve the desired effect with a mantra--he can cast a locating spell, for instance, by repeating "I want to find So-And-So." You can tell it's working because spellcasting causes at least his eyes to glow and if he wasn't flying before he started casting the spell, he'll begin to levitate anyway. Unless he's in a crossover, it seems. Now, this may simply be a matter of how much they can fit into a speech bubble, but it seems to take a lot more effort for him to, say, bring down Kang's shield, than it takes to cast a spell putting Patriot to sleep. Furthermore, his spells don't always work--he tries healing the Super Skrull's injuries once, and we get the light show and everything, but it's just a light show. Billy suggests that it might not work because even though he's going through all the motions, he doesn't
actually want to heal the skrull.
So. I figure it would be safe to say that his power is limited, if only in that it's not fully developed. Despite what the "Wiccan vs. Insert Name Here" discussions I've seen, I do not think it would be within his power to, say, wish someone dead. (Even if it was within his power, he wouldn't want to kill someone, so it still wouldn't work. ...Except perhaps under
extremely extenuating circumstances.
You touched Teddy?)
Billy seems to prefer playing a supportive role in the team anyway, typically using his powers to manuever things into a situation in which the rest of the team could easily neutralize the threat.
On a fairly unrelated note, the two Civil War storylines being released right now
both just had Billy get captured by the Pro-Registration forces. The YA/Runaways crossover takes place before the battle Billy gets captured in in the main Civil War storyline, though, and there's the possibility that Billy has been released and has escaped. I don't expect to find out anytime soon, though, so until otherwise demonstrated, I was planning on going with that. I'm assuming my twin will let me know if there's a problem with that!
Final note: Billy's username is based on a canon gay joke; Billy started his superhero career modeling himself after Thor, going so far as to make his codename "Asgardian." His teammates pointed out that since Billy has a boyfriend, he should change the codename before a certain pun becomes a "national joke."
And so, Billy is
ass_guardian.