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Ben Tennyson and Kai Green are one of the main couples in the franchise. At first, only Ben had a crush on Kai as opposed to Kai who only liked his alien forms. However, over time they developed feelings for each other.
History
Ben 10
They first met in Benwolf, Ben was smitten when he saw her performing a spiritual dance. Ben made attempts to impress her. However, at the end of the episode, when Ben tries to establish a relationship with her, she tells him that she only admired him as Blitzwolfer, where she could "train" and "tame" him, much to Gwen's annoyance and breaking Ben's heart.
Omniverse
In An American Benwolf in London, Kai's grandfather is taken away by the Forever Knights. She escapes and contacts Ben for help. Throughout their adventure to find her grandfather and the legendary sword Excalibur (of which Kai is proven later to be the only one worthy to wield it when she moves it), Ben and Kai exchange some banter. At episode's end, Kai kisses Ben on the cheek, much to his surprise.
Kai giving Ben a kiss on the cheek
In Fight at the Museum, Spanner reveals that he has come from a future where Kai and Ben are married. Due to his bluntness and immaturity, Ben aggravates Kai increasingly throughout the VIP tour, worrying Spanner. However, when Kai initially believes it was Ben who saved the Orb of Pooma Poonkoo, she embraced him without hesitation. Later, Spanner purposely locks Ben and Kai in a room together in the hopes of having them bond, but to no avail. After the orb is stolen, Ben is saddened to see Kai cry and tries to comfort her.
In Secret of Dos Santos, Kai has developed a love-hate relationship with Ben due to his antics and arrogance, particularly finding his nickname "the future Mrs. Tennyson," simultaneously annoying. She kisses XLR8 as her thanks when he reveals the location of the temple. When Ben collapses and Kai catches him, Ben calls her 'sweetie'. Due to being exhausted and injured from his fight with Exo-Skull, she also aids Ben all the way to his Time Cycle and asks if he was all right enough to ride home.
Ben & Kai's accidental kiss
In The Most Dangerous Game Show, Kai along with other girls Ben encountered is selected by Charles Zenith to participate in his show, where the girls try to win Ben's hand in marriage. When the final round commences she has no ride and so to help her Ben calls forth his Time Cycle. Kai comments on this and leans over him. Later, Ben chooses Kai over Ester to be the winner of the show. When Ester tell Ben that she does not want to fight destiny, Ben trips onto Kai, resulting in an accidental kiss between them. Despite this, the two blushingly smile to each other, instead being upset, finally coming to terms with their feelings towards each other.
Future
In the future, it is revealed that Ben married Kai and that they had a son, Ken.