I think the biggest piece of advice I can give you with this design, and also all your other designs, is that in manga you shouldn't be afraid of wilding up the hair a bit.
You seem to be very obsessive about the head being completely round, like a perfect circle, and any hair features are at the bottom. The top of the hair can vary, too! Look, most anime designs, even ones with realistic hair, don't go anywhere near as round as you go. It seems like you make their heads a perfect circle and then only slightly add some hair lines to clarify it's hair. All your characters look like they just got out of the shower and their hair is stuck to their heads too much, especially Hiro Yama. Aside from Yuka. For some reason Yuka has correct hair where even strands at the top exist.
this is just a random image I got by googling "anime girls", but look at the top of their heads. man, the hair has shape. your hair has no shape.
Anyway aside from that, accessorize, try doing things like giving her a special earring, or glasses, or a necklace, something that will make her different. If you see Naruto drawn in any style by anyone, a great artist or a 3-year-old, you will know it's naruto, because it will have the three whiskers at each cheek. That's the kind of thing that makes the character stand out. Give your character a feature that no matter what style she is drawn with, people will know it's her.
A character I designed a while back had feather earrings, that looked like this:
So, that's the kind of thing that makes your character special.
