Obtained: When Yashiro reaches Stage Rank 6, he will send you a Topic. After you read it, head to Fortuna Office and speak with him.
After you accept the side story, Mamori will send you an urgent Topic telling you that Yashiro collapsed in front of the cafe. Travel to Cafe Seiren in Shibuya and speak with Mamori who is standing near the entrance. I chose the second dialog options for both choices and it worked out wonderfully! After the cutscene, Yashiro will be ready to take on a new Radiant Skill. Head on over to Bloom Palace to unlock “Door of Hunger”.