ancient ordinary kimono
袿 refers to a specific type of ancient ordinary kimono, a traditional Japanese garment worn in historical contexts.
袿 combines the clothing radical 衤 with the phonetic component 圭, which likely indicates the sound. The character was created to denote a type of kimono.
The left side 衤 (clothing) and the right side 圭 (a jade tablet shape) together suggest a formal garment: imagine an ancient kimono decorated with jade-like patterns.
For ケイ, picture a key (ケイ) pattern woven into the fabric of the ancient kimono: key -> ケイ, and the key design adorns the garment.