It looks really good, I believe it will make your friend very happy :) One thing that I noticed is that the back of the head seems to have an unnatural amount of hair. If you do a draw-through for the skull you may found it being a bit elongated. Here is where I believe the hair should end.

Hey, you did a fine job! Without reference it's a bit hard to give a fully objective critique, so try to upload one next time you ask for advice :) As for the portrait itself, I believe the mouth should be a bit higher up and distance from it and the nose generally is quite a bit shorter. Aside from that, both lips look a bit too full and I think the nose should be facing more towards the direction her eyes are looking at. Here, I did a quick liquify so you can see what I mean, visually (Also check out the neck, ear and jaw). I hope it helps : )

Hey Sinix! What are some exercises for improving imagination drawing? (aside from studies lol)

This is a great start! The structure is feeling good and the markmaking is really lovely. As far as making the colors a bit more exciting; I would focus mostly on creating temperature contrast. If you darken the skin just a touch you'll have more room to get some lovely cool highlights in there, influenced from the background. Then I would keep an eye on any downward facing planes / shadow edges for opportunities for reflected light and subsurface scattering to create warmer tones for contrast. You can also push the cool tones even further if you want a more stylized finish; take a look at Steven Assael's work for some great examples. Also I'd keep an eye on so heavily separating the head from the neck; it's hard to say for sure without seeing the reference, but it looks like you have the jaw rolling into full shadow but its value is much lighter than the other shadows. I would probably darken it down and then bring the chin back out with reflected light. Great piece, keep it up!! :D

Great initiative! Huge thanks to Proko and the team!