Obtained: After achieving Stage Rank 7, you will receive a Topic with a new side story from Tsubasa wanting you to meet her at the usual spot in front of the cafe. Head on over to Cafe Seiren in Shibuya and speak to her.
After accepting the story, it’s time to begin your search for the cat! First, head to Shibuya Station and speak to the guy in a school uniform (Stray-Cat Enthusiast) standing next to the map near two other students. Next, head slightly west of Hee Ho Mart to find the Cat Aficionado, who is wearing a black jacket with a white stripe on the sleeve. Speak to him to learn that the Little Devil stays in the alleys around here. Afterwards, head back a short way east towards Hee Ho Mart to find the alley between where the Cat Aficionado is standing and Hee Ho Mart. When you near the alley, it will trigger a prompt to call Tsubasa and a cutscene will ensue. Afterwards, head on over to Bloom Palace to learn Tsubasa’s Radiant Skill, Door of Allure.