Taeruin's outfits are all green due to a lore reason. Partway through his comic, he encounters a deceased buck and removes it's antlers (bucks/deer are significant to Taeruin's backstory and elven culture), and so he drills a hole in one of the prongs and wears it around his neck for the whole rest of the series.
Lumideyr doesn't remove the scarves around his hips, because they're enchanted to act as his legs! If he removed 'em, he wouldn't be able to walk. So, regardless of what he's wearing (or if he's otherwise naked), he keeps his scarves on.
Ollee always has his screen around his neck or slotted into his chest plate. He's selectively mute, and uses it to talk sometimes. He also shows pictures on it, or little emojis pop up while he's speaking. It's removable, but it makes him anxious to go without it.
Placebo's TV-head helmet is probably the most literal example that I have. It's his signature in-series; he's a celebrity and public figure who's known for his eccentric public persona. Few people have actually seen his face. There's merchandise of him and his helmet, copycat celebrities, etc.
I don't have any pics uploaded with them, but most (if not all) of Gideon's outfits involve ballet flats or no shoes at all. His favorite type of outfit is also very short shorts with a sweater, so he wears that often. His signature color is purple.
Most of Eastrolach's outfits are blue, and most of Luna's are purple, but sometimes they wear matching outfits of the same color. They're brothers, and royalty, so they wear their kingdom's colors.
Eni has... a few signature items. The rose on his eye covers up what he believes is his rotting right eye (really he's just blind in it). He wears band-aids because he thinks that worms are crawling out of his skin, so he covers them up. And then, the ceramic hand on his head... well, I haven't decided why he wears that yet, haha.
All of Poppy's outfits are going to involve the color pink.
I'm sure there are more, but it'd take a long time for me to go through them, haha.