Shego

After a couple early episodes where it appeared Shego wanted to kill Kim it was revealed that Shego had the power to kill Kim easily if she wished. But she didn't. The two occasionally worked together, and by season four Shego saved Kim's cute little ass from danger more than Ron did.

Kim and Shego are one of the basic couples for KP fanfiction. That being said, the stories show a lot of variation. There are unrequited love stories. There are a lot of B&D stories, with Shego usually (but not always as the dom). And there are a lot of classic love stories where the dashing hero and the villain's female assistant fall in love.

The two often end up impregnating each other in KiGo fics. In one story it is real biology that exists and has produced animals with two mothers. More often pregnancy results from mad science or magic. (And a few of the younger writers don't seem aware of the role of sex in making babies and it simply happens.)