To get this achievement, you must unlock the Kira*Miki ending. On Day 2, purchase Mulan Tea from the shop and serve it to Kira*Miki (from the Bottled Drinks section of the menu) on Day 2 and Day 17. It's not clear if this is required, but to guarantee the ending, you should also make sure to flawlessly serve Sei, Stella, and Kira*Miku throughout the game. The ending will unlock after the credits.