Hayate no Gotoku!

Synopsis

It's a hot day in the Sanzenin mansion, Hayate is searching for an Ultimate attack, and Maria suggests looking in the Sanzen'in library for hints. They find a book "Sanzenin Secret Technique" that teaches the Sanzen'in secret ultimate attack that was used by Himegami. When Nagi is reading the instructions she read a lesson about being "lovey-dovey" but hides it from Hayate. After some training, Nagi finds out that Hayate is training for Hinagiku's sake and burns the book. Depressed, Nagi leaves the house at night where she meets Ayumu who challenges Nagi in a one-on-one match for Hayate in a karaoke box; jealous when Ayumu heard Hayate is now Nagi's butler. The result of Nagi's singing says 100%, which means she won. Ayumu tells Nagi that they will meet again in their next battle. Meanwhile, Hayate and Maria are still inside the karaoke box, with Maria singing her song.

(Source: Wikipedia)