A slang term most often used within internet forums, it's meaning is practically exactly the same as that of cool I.e. something awesome, great, teh shit or similar.
"Cool Guy Kai" or "CoolGuyKai" is a term coined by the LGBTQ+ community that refers to a specific dynamic in polyamorous relationships used to identify any adult straight person that pretends to be bisexual so they can lure teenagers in a relationship with them with the goal said teenagers also have sexual intercourse with their adult partner, who is an ephebophile. It is a term created to condense predatory behavior towards minors within the LGBTQ+ community.
"Don't be such a Cool Guy Kai. She just turned 18. We should date someone older."