I like the designs, I think they are pretty cute for the girl, and the guy does look neato, he reminds me a little of Adol from Ys with the way his armor looks.
Some things were bothering me. Copied the picture and gave it a quick whirl to see why I was bothered. I forgot to move the girl's left (our right) leg some to compensate for the changes, but whatever, it's drawn mostly correct.
Anyway, I'd really really really recommend changing where the guy's waist is currently, As you can see with the pink lines, he looks like he has a girl's body currently with where his waist begins. I know in anime they sometimes do this, but it's not a good habit to get into lol. I tried to show where I would start the new waist lines at with the blue lines (sorry if it is hard to see). The legs aren't too long or anything, they tend to be shorter on men, so hardly any changes there other than beefing him up in general. I did that because he's appears to be some sort of warrior, not a sneaky poor and hungry type thief.
On the girl, I'd just give her a bit more curvature, it's a personal taste thing. I did screw up a bit and forgot to move the leg right leg (her left) like I said above, but all you need to do is move it over closer to the other leg slightly to make her not look like she's falling down. I also added a bit more to her legs to not look like sticks. But again, that's a personal taste.
I know and realize this is a concept piece and you haven't started making the sprites/2d animated figures yet, so it's easiest now to make changes where you can.
I really like the designs and just wanted to give an opinion on how to make them better. Hopefully nobody thinks I'm being just a big fat party pooper on your art.
Edit: Oops found a couple typos..