This Demon King Is So Useless

Sister: "Your Majesty the Demon King, can you use magic?"

Demon King: (Magic explodes after failing for the nth time)

Guard: (Wipes the soot off the Demon King's face for the nth time)

Sister: "All right, magic is too advanced. Your Majesty, can you speak human language?"

Demon King: (Shakes his head)

Guard: "He seems to speak demon language."

Sister: "All right, not being able to talk is no big deal. But, Demon King, can you feed yourself?!"

Demon King: (The spoon falls onto the table for the nth time)

Guard: (Places the prepared spoon in the Demon King's hand)

Sister: "Forget it, I'm hopelessly disappointed. Demon King, surely you can get dressed, right?"

Demon King: (Wearing his boxer shorts on his head)

Guard: (Covers someone's eyes) "Stop getting in the way. You can get lost!"

Sister: "Bro! The Demon King you picked up is so useless!"

Guard: "As long as I like him." (Closes the door)

This is a story about a useless Demon King evolving into a Demon King. Probably?