Akina Iridatsu

Akina Iridatsu (伊里達 秋奈 Iridatsu Akina) is Yūta Iritatsu's sister.

History
Akina's grandfather found the unconscious Pine after he had swapped bodies with Chiyoko. Her family adopted the "girl" as "Yu Iritatsu", making her Akina's sister through adoption. However Yu, in fact being a boy in a girl's body, lived her life as a male, using a band to suppress her chest and going by the name "Yūta".

At some point while living at Korai House, Akina broadcast a 24 hour live-feed of her life to the online video sharing site Niconico. During this time, she also befriended Rabura Chichibu, and would apparently listen to her vent. She would later go on to study abroad in the US.

On December 16th, she sent a package to Yūta containing the Kenji Miyazawa costume which she brought at a party store.

Personality
That fact that she broadcast a 24/7 live-feedover the internet suggests that Akina doesn't feel embarrassed about sharing her private life with other people. It is inferred from both Rabura's jealously and the context of virginity and dating in which she mentions Akina's live-feed, that said live-feed may have included less then scrupulous moments.

It is unknown what she thought of her "sister" living her life as a boy. However in her letter she refers to him as "Yu-tan". The honorific "-tan" in the context it's used typically refers to a female family member. This combined with calling him "Yu" heavily implies she still sees him as a sister.