Tsukasa Sekoguchi is a friend of Yūichi Ezaki who visits him in the hospital. He assists Yūichi with secret missions to support Yūichi's relationship with Rika Akiba. For these operations, Tsukasa adopts the Zebra Mask disguise to conceal his identity and project strength or mystique. In one documented instance, he helps Yūichi retrieve Rika's valuable book from a high window ledge during adverse weather, requiring coordinated effort. His actions consistently create opportunities for Yūichi and Rika to connect or reconcile.