Obtained: During the Chapter 2 Intermission, accept a Topic from Tsubasa Oribe then head to Cafe Seiren in Shibuya to speak with her in person.
After you accept the side story, a dialog cutscene will occur in which you are trying to help Tsubasa lose her social anxiety. After the cutscene, you will receive a Topic from Tsubasa. Per her instructions, head on over to Shibuya Station to meet up with her again. Speak to Tsubasa near the crosswalks to begin coaching her. When you regain control of your character, head on over to Bloom Palace to obtain Tsubasa’s Radiant Skill of Door of Connection.