Ben Tennyson and Kai Green

Ben Tennyson and Kai Green is one of the main couples in the franchise.

Ben 10
They fist met in Benwolf, Ben was smitten when saw her dancing 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

In Fight at the Museum, Spanner reveals 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.

In The Most Dangerous Game Show, Ben trips onto Kai, resulting in an accidental kiss between them. Despite this, the two blushingly smile with to each other, not upset with the event, finally coming to terms with their feelings towards each other.