Really depends. Some characters I have a theme for or inspiration.
Hiro Hinmoku
Hiro Hinmoku (blue girl below) I wanted to create a superhero suit out of a mimic/slime creature, but I didn't want it to look like armor. So I went with normal clothes and added a vizor to give it more of hero look plus leaving the face exposed just felt a bit off. It's not a glass vizor it's the slime/mimic substance solidified to form a glass like appearance.
Hair Style was based on Yumi from Code Lyoko
Cyan and Blood Parasite
Blood parasite I tried to give a unique appearance. I thought about his abilities which lead to me adding a ring around the body. As for body structure, since it was made from blood I thought it should look like a slime. I suddenly thought of slugma from pokemon and experimented with the head to give it my own touch. I thought about a cell/bacteria and made those attena from the head to symbolize that.
As for Cyan I was a bit inspired by Yuno from black clover in terms of hairstyle. In my story people have color words for names like Azure, Cyan, etc. So I made him wear blue, but he's also a wizard in training so I gave him wizard clothes of my own design after looking at many different styles.
(Left to Right ) Aniro, Den, Anaga
Aniro's personality is more gentle like gohan, deku, tanjiro, etc. So I associate him with green (those characters are associated with that color) and created combat attire similar to Deku's. All 3 were inspired by scrapped designs of Naruto, Sasuke, and Sakura. I gave Den similar attire to scrapped Sasuke design with my own touches. A lot of rival characters wear blue, like vegeta, sasuke, Killua, Grey from fairytail so I associate Den with the color blue. Anaga I actually just experimented with many random designs. Her clan specializes in waves, so her outfit has rings/stripes to symbolize that. Some female characters are associated with red, like Erza, Sakura, Yoko from Gurren Lagan, etc. So I associate Anaga with the color red in terms of attire.
