Kai Green

Kai Green is Ben's first love interest, who makes her appearence in Benwolf. Kai is the granddaughter of Wes Green, a friend and former partner of Grandpa Max. She first appeared doing a spiritual dance in the episode Benwolf. She was Ben's first crush, but she only took interest in him after he started becoming Benwolf because she wanted to, as quoted, "train and tame him". She did have an interest in him before he turned into Benwolf but the interest was about the Omnitrix. When she admitted to this, Gwen gave her a stern talking too. She made a cameo appearance in Ben's dream world in Perfect Day (though it was only a dream).

In the alternate future including Ben 10,000 and Ken 10, it is confirmed that Kai is Kenny's mother. Albeit, we are not told how or when these two worked out their differences from Benwolf, nor does the episode show or mention Kai's whereabouts, until the Ben 10 pop-ups, which confirmed she was on a mission in the shadow realm. Then again, this future does not exist in the Alien Force/Ultimate Alien time line because it is the non-canon future and it will never happen.

Original series

 * Benwolf
 * Perfect Day (dream)