Liking it so far
I'll say off the bad that I'm not a huge fan of superheroes or superhero stories in general. But I gotta say that there are a few that manage to pull me in and care about the characters. And that's something you managed to do so kudos on you for that!
Read the story now
So, I like the overall plot of this story. It's actually very simple and something most (from what I've read/seen) superhero material does. You have an interesting villain in Doctor X and you have interesting superheros (so far) that are different than most.
David - is from a different universe. Reminds me of Thor in a way, but I enjoy his humour. The only thing I wish is that we could have a bit more background on him and his original universe and what exactly happened to have him in ours.
Leah - reminds me a lot of Twilight. I'm assuming that you were inspired by the novels for the background and I do love me some werewolves. I like the twist you put on how she transforms. But one thing, does she need the full moon to be able to change or can she do it at will? That's something that confused me when reading her story.
One thing I'll admit to not being fond of is the villains of the hero's personal arcs. To me, they just don't seem like villains, but very horrible people (and disgusting) people.
I will say that I wish you'd expand on the character's history since they really are diverse people and cultures. I also want to know a lot more about Doctor X, and if we do, then I hope you explain how he became the way he is. Or what his reason for targeting Earth is.
Overall, I'm enjoying this story and can't wait to meet the next superhero in this saga! Good work :)