fragrant, balmy, favourable
馨 centers on a pleasant, spreading scent: something fragrant or balmy. The favorable sense extends naturally from the idea of a good, agreeable aroma that creates a positive impression.
馨 combines 殸 (a component associated with sound or resonance) with 香 (fragrance), suggesting a fragrance that spreads or resonates. The exact historical development of 殸 is uncertain, but the combination with 香 clearly points to a spreading scent.
The top 殸 looks like a resonating instrument, and the bottom 香 is fragrance. Imagine a bell that, when struck, releases a wave of sweet perfume: the scent spreads far and wide, creating a favorable, balmy atmosphere.
For ケイ, picture a fragrant cake (ケーキ) that fills the room with a balmy aroma: cake -> ケイ, and the scent is so favorable everyone wants a slice.