Apart from Kaguya, Shirogane, the rest of the student council, and the pink-haired agent of chaos in Kaguya-sama: Love is War, the other popular character would be Kaguya's maid, Ai Hayasaka. She's so likable, some fans even prefer her to the main character as a potential romantic partner for Shirogane.
Too bad she mostly loses the spotlight to the titular character. But for those who just can't get enough of Herthaka — sorry, Hayasaka-kun, then it's best to do her some honors by knowing more details about her life. Worry not as this doesn't count as a stalking tendency. Not yet.
The last two entries contain spoilers for those who haven't read the manga.
10 She's One-Quarter Irish
One physical trait that immediately stands out with Hayasaka is how she has blonde hair whereas everyone else (save for that habitual cheater Fujiwara and Shirogane himself) has the typical black or brunette Japanese hair. This was intended since Hayasaka is also of Irish descent.
It was revealed on a character page in the Volume version of the manga. She's only a quarter Irish, but apparently, that's enough to affect her physical appearance and make her stand out compared to the other characters. Apart from this she also has some striking blue eyes.
9 She Wears Contact Lenses
Speaking of Hayasaka's eyes, one of the reasons why they also stand out is due to corrective contact lenses. Much like Miyuki Shirogane, her eyesight isn't exactly perfect and instead of wearing eyeglasses, she chose the trouble of contact lenses.
It's not clear whether her blue eyes are just from contact lenses or are natural, but the contacts are part of Hayasaka's strict daily awakening ritual. Other activities of this ritual include putting on makeup, fixing her hair, and brushing her teeth.
8 It Was Kaguya Who Asked To Be Friends
Kaguya and Hayasaka have a long history (about a decade) together and they're practically sisters. They're not blood-related, of course, and Hayasaka was merely introduced as a servant to Kaguya at the age of seven. Of the two, it was surprisingly Kaguya who initiated the friendship.
One would think that it was Hayasaka who had to tear down Kaguya's walls, but back when they were just children, it was the latter who was more innocent and friendly. Kaguya offered a hair tie and formally asked to be friends with Hayasaka. That was the start of something beautiful.
7 She's Juggling Four Identities (Maybe More)
It's well-known in the manga and the anime that Hayasaka isn't being true to herself and to the people around her. She mostly masks her personality with well-crafted and well-acted personas depending on her activity or her short-term goals.
Currently, she has four personas and retains two or three of them, switching them up depending on her environment. Of course, knowing her set of skills and capabilities, there might be more, especially if Kaguya "requested" some odd and ambitious tasks.
6 She Has A Rare Blood Type
In another of the character pages in the manga, it was revealed that Hayasaka has one of the rarest blood types known to man, which is the AB group. It's positive so not really as rare compared to its negative counterpart but it still reinforces Hayasaka's peculiarity.
The blonde hair, numerous alter egos, tech-savviness, and now her blood type makes her one of the most memorable "best girl" candidates in Kaguya-sama. Still, having that kind of blood isn't a detriment in a medical emergency since Hayasaka's family is still astonishingly rich and powerful.
5 Her Character Is Based On Another Servant Character
Much like some of the characters in Kaguya-sama, Hayasaka is also based on an older story from Japanese folklore about a princess from the moon named Kaguya-hime. It's a classic Japanese prose narrative with a Studio Ghibli film adaptation.
Hayasaka's character is based on a servant from Princess Kaguya's "The Tale of the Bamboo Cutter" folklore, which is also known as "The Tale of Princess Kaguya." But this servant sadly isn't Kaguya's loyal servant, but an emperor's servant.
4 She's Had Arrested Development Thanks To Kaguya
From the beginning of her childhood, Hayasaka was trained to become Kaguya's servant but really, it was more of a parenting job. She's the one who taught Kaguya how to be a human being in lieu of Kaguya's late mother. Oftentimes, she ends the day exhausted and drained.
The sadder part is that she's just a teenager and for the most part, couldn't enjoy her teenage life due to her duties for Kaguya. While she's shown to have friends in high school, the gyaru idol persona was just Hayasaka's front to keep tabs on Kaguya. What she longed for the most was to be liberated from servitude and to enjoy her life the way she wanted.
3 Her Savings Are Higher Compared To Other Characters
Among the high school characters in Kaguya-sama, Shirogane and Hayasaka were among the few to have actual jobs. Both were equally hardworking and burnt out, but Hayasaka was better paid since she worked as a maid for the Shinomiya estate.
The tasks she had to endure and perform were also absurd from time to time so those likely warranted extra compensation. In the manga, her estimated savings are somewhere around 40 million yen, making her the wealthiest financially out of all the main characters in the anime (inheritance and material possessions notwithstanding).
2 She Cut Her Hair
In one of the most eventful arcs of the manga, Hayasaka cut her hair after being held by the ponytail by one of the Shinomiya servants. Such an act was seemingly a symbolic parallel for Hayasaka severing her bonds of servitude and her burdens.
This was at the same time when she left her servant job at the Shinomiya estate (more on that later). In any case, it also marked a new look for Hayasaka. Now, instead of the side ponytail, she now sports a medium-length bob cut, arguably making her cuter. One could also argue that this was Hayasaka at her most honest phase.
1 She's Not Exactly Being Honest With Kaguya...
The biggest event in Hayasaka's life was after she finally caved in from the pressure of having to betray Kaguya every single day since she became her servant. It turns out, Kaguya's estranged older brother wanted to spy on her so he used Hayasaka as his informant for a decade in order to gain something he could use to exploit his younger sister.
Hayasaka at seven years of age initially went under the guise of sisterhood and friendship in order to gain Kaguya's trust but this pretense quickly became genuine although she still carried on with her spying. Thankfully, the bond between the two remained strong and unfaltering.