Tsukasa Hiiragi (柊 つかさ ,Hiiragi Tsukasa?) is the younger fraternal twin sister of Kagami Hiiragi and lives in a six-member family household. She is in the same class as Konata Izumi. She isn't good at studying or at sports, but excels in cooking. Tsukasa often depends on Kagami for help on her homework, like Konata, but their reasons are different: Tsukasa easily forgets her assignments, while Konata is just too lazy or preoccupied with her hobbies to do them herself. Unlike some of the other characters in the series, Tsukasa has not shown any particular interests in video games and anime, but she does like the Sgt. Frog series.
Tsukasa's physique is a little smaller than average. She has short, purple hair with a ribbon like a head band in it. Her eyes are droopy and she is portrayed as a stereotypical klutz. Tsukasa once received a mobile phone for her birthday, but didn't have much luck with it, as the phone was taken away by using it in class and later mistakenly ruined by putting it in the laundry, along with her skirt. Once, she left instant yakisoba alone after pouring hot water in it, only to come back later discovering there was no water left to drain. Unlike most of the cast of Lucky Star, Tsukasa is a heavy sleeper, and cannot wake up at a set time, even when someone is present to do so. Like her sister, Tsukasa is left-handed.
She can be mistaken for cosplaying as Akari Kamigishi from To Heart when she wears her autumn school uniform, which is pointed out twice in the anime: once, when Konata gave her the actual uniform used in the To Heart anime for her birthday, and the second, when two people stopped her at Akihabara to take her picture, much to her embarrassment.