Super Sayian 5 Goku(Ssj5 Goku)

[Silverthorn]

what to improve? I think the hair looks good now

May I say something towards this? There's still room for improvement left. Also on the hair. There are two colors; you should have at least three: basic color, highlight color, shadow color. Furthermore, it really helps if you blur the transitions of the colors a little. They're definitely too sharp.
Also, the left arm (from our perspective the right one) looks a little too much sticking out. Like if it's hovering in front of the character.

I made it as detailed as possible

Good to know. However, sometimes less is more. Means, you shouldn't go crazy with the details. A simple, well-shaded character is ways better than a detail-overloaded not-so-well-shaded one. I hope you get what I mean.

Keep on improving.