mouse
鼷 refers to a mouse, specifically a very small kind of mouse or a young mouse.
鼷 combines 鼠 (mouse) with 奚, which likely contributes a phonetic element. The exact historical development is uncertain.
The left side is the mouse radical 鼠, and the right side 奚 looks like a tiny creature with a big head and small body. Picture a very small mouse scurrying around.
For ケイ, imagine a tiny mouse named Kay who squeaks 'Kay!' whenever it appears: Kay -> ケイ.