water caltrop
芰 refers specifically to the water caltrop, an aquatic plant with distinctive horned nuts.
芰 combines the grass radical 艹 with 支, which likely contributes a phonetic element. The exact historical development is uncertain, but the character is used for the water caltrop plant.
The grass radical 艹 sits above 支, like a plant supported by a branching stem. Picture the water caltrop's floating leaves and spiky nuts branching out from a central point.
For キ, imagine a key-shaped water caltrop nut unlocking a pond: key -> キ, and the key-like nut is the plant's distinctive feature.