Obtained: During the Intermission after Chapter 1 ends, open a Topic with Touma then meet Touma on Central City Street in Shibuya.
Head to Toubo Rooftop and speak to Touma in the north side of the area. After a cutscene, you will receive an urgent Topic from Touma. Head to the Idolasphere in Daitama Observatory and use Touma to defeat 5 Blobs. Touma’s Door of Rampage Radiant Skill will then be unlocked so head to Bloom Palace to receive it and trigger a cutscene, which will complete the side story.