Kai Green is one of Ben's first love interests, who makes her first debut in Benwolf. Kai is the granddaughter of Wes Green, a friend and former partner of Grandpa Max.
Kai has considerable tracking skills taught to her by her grandfather. In Omniverse, it is revealed that she has been taking Judo lessons, showing considerable skill against trained soldiers of the Forever Knights.
She first appeared doing a spiritual Native American dance in the episode Benwolf. She was Ben's first crush, but she only took interest in him after he started becoming Blitzwolfer because she wanted to, as quoted, "train and tame him". When she admitted to this, Gwen gave her a stern talking to.
Kai appeared in An American Benwolf in London. Working with her grandfather, they find the secret laboratory of Dr. Jekyll, quickly afterwards ambushed by the Forever Knights in their search for Dr. Jekyll's Hyde Formula. While capturing Wes, Kai manages to escape and contacts Ben and his partner Rook for help. Ultimately, they discovered the hidden location of the legendary Excalibur. When removing the blade from its stone seemingly proves impossible even with the Hyde Formula or Ben's strongest aliens, Forever King Chadwick orders his men to kill their enemies. As Ben battles the mutated Forever King, the cave's underground pipes were ruptured and caused a flooding. Convincing the Forever Knights to work together, everyone was able to escape alive.
While still greatly liking Blitzwolfer, she has shown signs of growing feelings for Ben as himself, becoming bashful when people imply they are a couple and giving Ben a kiss on the cheek as he leaves.
Kai Green's outfit bears a similarity to Lara Croft from Tomb Raider.
Kai considers Blitzwolfer to be her favorite out of all of Ben's aliens.