I'm using Ki's post-timeskip form as the header for a reason. This Fact is revealed to Ki herself during the two years and then She reveals it to the rest of the crew.

Ki could be considered a half-breed in a sense. She is half human, half elf. Her father, Ken, was human while her mother, Silver, was an elf.

Ki finds herself in her mother's birthplace for the two year skip. While there, she naturally trains her swordsmanship, and also learns to use the elven magic that is in her blood. Defensive spells, offensive spells, healing spells, elemental spells, summoning spells, she learns as much as she can from the elves that raised her mother.

I gave Ki this background because, let's face it, Zoro and Ki can't keep their hands off of each other, so naturally, eventually Zoro gets Ki pregnant. But, just like Zoro, Ki hates being left out of the action. So, her elven magic allows Ki to still be in the fight from a safe distance, keeping her and the child/children she carries out of harm's way, much to Zoro's relief.