Kai Green is the future version of Kai Green.
Some time after Ben 10,000 and prior to Ken 10, Kai married Ben 10,000 and had a son, Ken Tennyson.
It was revealed that Kai was on a mission in the Shadow Realm during the events of Ken 10, so she missed her son's 10th birthday.
In Fight at the Museum, Kai found the Orb of Pooma Poonkoo in Ben 10,000's tower that she thought was lost forever.
In The End of an Era, Kai helped Humungoopsaur fight Dr. Animo and later on, Maltruant. It is shown that she has Excalibur in her possession and she uses it as her primary weapon.
Ben 10: Omniverse