beautiful girl
姝 centers on feminine beauty, specifically a beautiful girl or woman.
姝 combines 女 (woman) with 朱, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound for a beautiful woman.
A woman 女 beside 朱, a shape associated with vermilion or red, like a beautiful girl with rosy cheeks.
For ス, imagine a beautiful girl named Sue: Sue -> ス, and her beauty is the meaning of 姝.