samurai topknot
髻 centers on a traditional hairstyle where hair is gathered and tied into a topknot, specifically the samurai topknot.
髻 combines 髟 (hair) with 吉, which likely contributes the sound. The character refers to a topknot or bound hair style.
The top part 髟 represents long hair, and the bottom part 吉 looks like a knot tied neatly. Picture a samurai gathering his long hair and tying it into a topknot on top of his head.
For ケイ, imagine a samurai named Kay tying his topknot: Kay -> ケイ, and his hair is styled into a 髻.