Sakuya Aizawa is Nagi's cousin and from Kansai. Like Nagi, Sakuya is very rich and has a similar personality — loud, arrogant, abusive of servants, and unwilling to accept defeat in anything; still, she has some kind aspects, like looking after lone Nagi since she was young and giving up her admission into Hakuou for Wataru, encouraging him in his feelings towards Isumi. Nagi is one of the few people who can stand up to her wrath, and out of Nagi's friends, Sakuya is the one who has known her the longest. Sakuya is the eldest daughter in her family with three younger sisters and one younger brother. Her illegitimate half-brother Gilbert is one of the people who is constantly harassing Nagi for her fortune.
Sakuya is obsessed with stand-up comedy, especially Manzai-style. She encourages Hayate to become a comedian because she believes that Hayate's life up to now is nothing more than a comedy.
In volume 14 of the manga, Sakuya confesses to Hayate that she wants a proper older brother, as she does not respect her half-brother Gilbert in that manner at all. After attaching the suffix -oniichan after his name, she promptly gets embarrassed and goes back to addressing him normally again.
Sakuya can often be seen hanging around with Isumi: she is usually present when Isumi performs exorcism, although she doesn't have any special powers. In volume 21, when Isumi disappears after being seen in Greece with Nagi, Sakuya flies to Greece to look for her: later on, Isumi and Sakuya reappears when Hayate engages Athena possessed by King Midas, but it is unknown how they found the place or about the events involving Athena and Hayate.