Making a default 'gacha template' XL twin tails, huge shiny eyes, blush, and a million accessories — that's the look that makes everyone seem like the same OC. It's fine as a test base, but your memorable OC changes ONE big thing: an unexpected skin tone, a different hair style, or a rare accessory. Break one rule.
Overloading with accessories Every accessory competes for attention. A character with wings, halo, crown, sword, backpack and necklaces reads as noise. The 2-3 rule applies here too: choose one star accessory (the silhouette) and one supporting piece, tops. Keep a 'no accessories' version saved as a clean template.
Using too many colors Five different tones across hair, eyes, skin and clothes make a muddy design. If you can't name the palette in one phrase ('red and black'), there's too much. Trim down to 2-3 dominant colors and one accent.
Ignoring the face's emotion Your OC's story is told by the face. A demon with a wide smile and curious eyes isn't scary; an angel with a flat mouth and narrowed eyes doesn't read good. The eye and mouth shapes must match the emotion you set in step 1.