I see you use shading on the necks (and foreheads), but not on the hair. You should either shade the hair, or remove all shading.
The line that divides the hair and the face is too dark in my opinion. It's justified for the one between the head and the neck (because it's a dark area), and the outline, but not for the hair-forehead line.
The eyes sort of look like they're pointing outwards, either because the outward upper eyelines are square instead of round, or because the light that makes their eyes glow seems to come from the center of their faces.
To fix this, make the shiny upper part completely horizontal. Or, as that screws up the eye outline, only do one pixel (the middle one). The important things is that it doesn't look as if the light comes from their forehead.
Also, using exactly the same eyes in each and every portrait is sort of lazy (sorry if that sounds harsh)
It's obvious you take inspirations from anime, but the colors in the mouth lines feel too clashing to me. It sort of looks as if their mouths have been sewn in
You don't have to abandon the technique, but you should make the color differences not so abrupt.
Jul has no shoulders (are they behind her hair? If so, you should convey it better, using either lines or shading)
Neither Gia nor Abigail have eyebrows. I don't know them, so I don't know how accurate that is.
Hope I was of help.
