My Sister's Hero is Her Big Brother! (妹のヒーローはお兄さま! Imōto no Hīrō wa Onīsama!) is the 4th episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Ikusu Mizutani and Shougo Mikadono discussed about the cellphone which the latter's little sister used to call him. They came to the conclusion that the phone was a sophiscated and water-resistant device. Ikusu promised to continue her investigation to Shougo.
The following day at school, things got awkward between Shougo and Miyabi Kannagi, which brought concern for Konoe. Konoe cornered Shougo on the rooftop and made him confessed to her that Miyabi mightbe his little sister.
Shougo received a call from his sister, who only said "Goodbye" cryptically. Ikusu later contacted Shougo to inform him of a prototype Kyoasu cellphone never released to public, which had voice-changing capabilities, and might have connections to the calls he receives from the mysterious sister.
After recalled having seen Konoe having a similar cellphone, Shougo searches everywhere for her, finally finding her at a park holding the phone. Konoe revealed that she was the one kissing him in the photo he received, explaing that they were childhood friends when his father, who was also Miyabi's father acquaintance, was alive then. Shougo injured his head while saving her at a crosswalk from an oncoming car, which cause him to forget about her. Konoe used the altered voice feature from the prototype phone, which were the creation of her father, as a prank and did not realizing how much trouble she has caused for Shougo. The boy however readily accepted Konoe's apologize.
When Miyabi foundthe two, she revealed that she and Shougo were also childhood friend, and once had a pretend wedding in the courtyard of the hospital when she visited her father in the hospital where Shougo stayed after the crosswalk incident. Since both Konoe and Miyabi wanted to become his wife, they decided there to start a competiton between them to see who would be the better wife.