Number 9001 (ナンバー 9001) is a character created by Soaker87, who appears in a fan-made visual novel game from the Shounen Toppa Bashin universe.
Number 9001 has grey-blue eyes and brown hair, which she ties up in a ponytail. She wears a purple shirt, blue jeans, and a black cape. She has red-frame glasses.
Extremely delusional, she seems to believe that the rules are below her, and is a known cheater. She is obsessed with the Numbers. Her behavior can come off as creepy because of this, and it doesn't help that she's a master of skewed logic which she uses to try to justify things. She has a tendency to act paranoid. In short, she has a severe lack of redeeming qualities.
Number 9001 is a former-card battler who moved to Japan for the sole purpose of becoming a member of Numbers Elite, and ultimately "romancing a hot Number." Her definition of "hot" differs greatly from most people's. Generally, the known Numbers, if they even know of her existence, tend to just ignore her though.
She notably has a worse sense of direction than Number Eight (she once crossed an ocean when trying to get to a place around the corner) and is a terrible cook. She doesn't need to cook much though, because she lives entirely off of store-bought mochi, her favorite food.
In the past, she wanted to use a deck composed entirely of X-Rares when only BS01 was released. Because her deck used too many copies of the same cards, it was confiscated for breaking the rules. Number 9001 no longer battles, and is banned from ever entering the Battle Spirits Centers, so she has no way to buy new cards. Because of the low-profile she's forced to keep, her existence is generally unknown to the public. She also doesn't reveal her real name to anyone.
As is probably obvious, the character originated as a joke, and the name is taken from the "Over 9000" meme.