Tsukasa Hiiragi is the younger fraternal twin sister of Kagami Hiiragi and the youngest member of the six-person Hiiragi family, residing in Washimiya, Saitama Prefecture. She possesses a petite build, standing 157 cm tall, with light purple hair often adorned by a yellow headband featuring a bow. Her droopy, blue-violet eyes give her a gentle, dreamy expression. Tsukasa typically wears her school uniform: a loose white blouse with salmon-pink cuffs and collar, a matching short skirt, white calf-length socks, and brown loafers. This look occasionally causes her to be mistaken for Akari Kamigishi from *To Heart*, especially in her autumn uniform.

Her personality is sweet, naive, and gentle, contrasting her sister Kagami's tsundere nature. Inherently kind and non-confrontational, Tsukasa seldom gets angry, fitting a "healing-type" archetype. Academically, she struggles significantly due to forgetfulness and an airheaded nature, frequently needing Kagami's help with schoolwork and tasks like texting. She particularly dislikes physical education and performs poorly in sports. However, Tsukasa excels in home economics, demonstrating considerable skill in cooking and household chores, surpassing Kagami in these areas.

Tsukasa is a heavy sleeper who finds waking up at set times difficult, even with assistance. She enjoys simple pleasures such as strawberry shortcake, sleeping, and using mobile phones, while disliking horror films, waking up early, and strenuous activities. Her interests include series like *Sgt. Frog*, though she lacks broader otaku tendencies. She uniquely addresses Konata Izumi as "Kona-chan."

In the spin-off *Miyakawa-ke no Kūfuku*, Tsukasa appears in supporting cameo roles. She interacts with the main characters, Hinata and Hikage Miyakawa, showcasing her compassionate nature, such as offering food to Hikage. Her involvement remains consistent with her established gentle and supportive personality without significant narrative development.