Does repeating the name "Ya Kafi" – which is said to be one of the Idrisid names and related to magic, and one fears harm from its servant upon ceasing its recitation – cause harm, and what should I do to avoid that?

Al-Kafi is one of the names of Allah Almighty, and supplicating to Allah with this name is permissible, such as saying: "O Al-Kafi, protect me from the stratagem of my enemy." However, repeating the name in isolation with a specific intention has no basis in the Sunnah. What you mentioned about this name being one of the Idrisian names, or that jinn will harm you if you stop repeating these words, is all falsehood and superstitions that should not be believed. Rely solely on Allah and trust in Him, for He is capable of all things. Strive to supplicate to Him with His beautiful names and sublime attributes.