Kai Green

Kai Green is a Navajo girl who 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 Max Tennyson. She is also currently Ben's girlfriend.

Appearance
In the original series, Kai had long, black hair and brown eyes. She wore a red shirt, green shorts, brown boots with white socks. She had silver bracelets and a blue ring.

In Omniverse, Kai has grown noticeably taller. She is slender yet athletic in build. She wears a long sleeved red shirt with a red and peach colored sweater. She still has brown boots (now with blue stripes) and her shorts are even shorter now. Her hair is in a high ponytail and she also has a tan earring. She wears brown gloves, tan socks, and a belt.

Ben 10
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.

She made a cameo appearance in Ben's dream world in Perfect Day.

Omniverse
Kai appeared in An American Benwolf in London. Working with her grandfather, they find the secret laboratory of Dr. Jekyll, but are quickly afterwards ambushed by the Forever Knights in their search for Dr. Jekyll's Hyde Formula. While they capture Wes, Kai manages to escape and contacts Ben and his partner Rook for help. Ultimately, they discover 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.

In Fight at the Museum Kai comes to Bellwood to host a new exhibit on alien artifacts at the Bellwood Historical Museum. Her decision to invite Ben paid off when an alien thief named Subdora tries to steal one of the artifacts, one of the first that Kai found with her grandfather. Fortunately Ben's, Rook's, Ester's and Spanner's efforts foil Subdora's efforts.

Knowing that Subdora will be back the four team up and stay overnight with Kai, Spanner being forced to reveal that he comes from a future where Kai and Ben have been married for years. Thanks to Spanner's efforts Kai and Ben are locked in a room together where the two, despite their best efforts, bond. Later after Subdora's partner Exo-Skull comes in and tears through the museum and takes the "Key to time itself" from the artifact. Spanner takes the artifact itself, leading Kai to believe she has lost one of the most important things in her life.

In the Secret of Dos Santos Kai calls Ben to South America to help her track down The Temple of the Sky, an ancient temple with distinct alien origins. Kai soon develops a love-hate relationship with Ben with his antics, particularly his nickname "the future Mrs. Tennyson," simultaneously annoying and helping her locate and get into the temple. Arriving at the temple's treasure Temple Kai encounters a Galvan named Ujin.

Professor Paradox comes in and explains that the artifact the group has found is the final component of a villain named Maltruant, a mutant Chronosapien hellbent on taking over all of time and space. Exo-Skull tears into the room and with Subdora and Maltruant's help escape. Ben leaves Kai on a good note.

In The Most Dangerous Game Show, Kai is one of several contestants chosen to compete in "The Most Delicious Game" for the chance to marry Ben. She is the ultimate winner, and joins Ben on a mission to free the losers, who have been trapped in the Null Void. After Charles Zenith is defeated, Ben slips on a banana peel and falls towards Kai, and the two accidentally kiss.

Powers and Abilities
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.

Ben 10

 * Benwolf (first appearance)
 * Perfect Day (dream; cameo)

Ben 10: Omniverse

 * An American Benwolf in London (first re-appearance)
 * Fight at the Museum
 * Secret of Dos Santos
 * The Most Dangerous Game Show 

Etymology
In Navajo, Kai means "Willow tree".

Trivia

 * In the future, Kai will marry Ben and will have a son known as Kenneth Tennyson.
 * In An American Benwolf in London, Kai is shown to be able to slightly move the sword Excalibur from its stone, having grabbed on it to prevent herself from drowning. In the future it is revealed that she is indeed worthy.
 * Blitzwolfer is Kai's favorite out of all of Ben's aliens. She also thinks that Pesky Dust is cute.
 * While originally only interested in Ben's Blitzwolfer form, she has since shown signs of growing feelings for Ben as himself, becoming bashful when people imply they are a couple. She also on impulse kissed Ben on the lips during Secret of Dos Santos as thanks for showing her the way to the Temple of the Sky.