Kai is the granddaughter of Wes Green, a friend and former partner of Grandpa Max. She first appeared doing a spiritual dance in the episode "Benwolf". She was Ben's first crush, but only took interest in him after he started becoming Benwolf because she wanted to, as quoted, "train and tame him". When she admitted to this, Gwen gave her a stern talking to. She briefly reappears in Ben's dream world in "Perfect Day" as a cheerleader. On April 13, 2008, the enhanced version of "Ben 10,000" says that she marries Ben and had two children named Ken (named after Gwen's older brother who Ben looks up to) and Gwendolyn (most likely named after Gwen). She was not seen in "Ken 10" due to being on a diplomatic mission in the Shadow Realm, meaning that she is a Galactic Diplomat.

Kai Green's First Appearance
Benwolf (Episode)

Kai Green's Last Appearance
Perfect Day