servant, what, why
奚 centers on a subordinate or low-status person, hence a servant. The interrogative senses 'what' and 'why' likely developed from a grammatical use of the word for servant, but the exact connection is unclear.
奚 originally depicted a person with a hand (爫) and a rope or thread (幺), suggesting a captive or servant. The interrogative use is a later phonetic loan, not directly related to the visual components.
The top 爫 looks like a hand grabbing someone, and the lower part suggests a tied thread. Imagine a servant being held by a rope: 奚 means servant.
For ケイ, picture a servant carrying a heavy tray and saying 'Kay!' as a sigh of exhaustion: Kay -> ケイ.