Nonexisty does not exist and therefore has no appearance or personality. However, he still has traits mentioned throughout BFDI that suggests some level of existence, despite supposedly not existing, Ruby and Two refer to him with masculine pronouns, implying he's most likely male. He has a presence, as Ruby tells the Announcer that he missed him when attempting to fling him to the LOL.
Nonexisty had his own TikTok where he did stand up comedy.