Satō Matsuzaka is the main heroine of Happy Sugar Life. She learns to love for the first time after meeting Shio Kōbe and lives with her in a shared apartment. Publicly, she tells people that she resides with her aunt in her apartment, and keeps the fact that she lives with Shio a secret.
Satou is a freshman at Makikou High School and primarily works at Cure á Cute, maintaining a kind nature and getting along well with other people in order to sustain her lifestyle with Shio. While she wishes she could spend her time with Shio forever in their apartment, she is aware of the fact that they cannot live on peacefully if she does not work and earn money for food, utilities, etc. Because of this, Satou often has to confront the outside world where people interfere with her relationship due to her past history of accepting anyone who has confessed to her and changing partners every three days. Satou is prone to severe animosity when people disrupt her relationship with their interpretation of "love", and can be easily provoked when it comes to Shio due to her overprotectiveness. She is willing to take any action necessary to eliminate threats to Shio and their Happy Sugar Life.
Satō is a beautiful fair-skinned girl with hip-length light bubblegum pink hair that is tied into two small buns with a red ribbon with pleated white, a light red cross/diamond patterns on her right bun while she leaves the rest of her hair down, her bangs are parted to the middle and it has 2 strands of hair with the long ones nearly reaching down her chest with short choppy strands which are chin length and slightly curl inwards and ominous blood-red eyes. She also wears a lacy white headband tied around her left bun.
She is usually seen wearing her school uniform, which includes a white long-sleeved button-down shirt with a pale red bow with thin darker red stripes going to the left in a sideways position, a brown vest with two golden buttons, a navy blue jacket-like blazer with yellow collars and white stripes around the edges, and a thin white striped dark green skirt along with black knee-high socks and brown penny loafers. At home, she wears her white button-down shirt on its own with undergarments and has her hair completely down whenever she is in bed. Her eyes often depict the mood she is currently feeling and can change in shade depending on the intensity, for example, sparkly pink and red eyes indicate happiness whereas eyes with dark black and red swirls around the pupil can indicate indignation or animosity.
Around her co-workers and classmates, Satou is a nice and sweet girl. She is loved and admired by many for her ability to quickly adapt to any situation, her skillfulness in handling problems, and her beauty. Before meeting Shio Kōbe, Satou was a permissive girl, allowing men to have their way with her and consequently gaining the reputation of being "easy to get". She often changed partners in her pursuit of experiencing "love" and "completeness", which was greatly influenced by her aunt.
After meeting Shio, Satou still acts kind around others in order to sustain their lifestyle, however, she no longer pursues relationships or allows people to pursue her due to her devotion to Shio and how she made her feel "complete". Because this, Satou develops a reliance on Shio to fill her life with happiness and "sweetness". Her reliance causes her to be extremely obsessive and protective of Shio and their relationship. Without Shio, Satou becomes immensely sad, empty, and heartbroken, doing whatever it takes to restore their relationship and even going so far as to kill anyone who threatens her "Happy Sugar Life". Due to her aunt's skewed and irregular teachings of what love means, any resemblance to her aunt's ideologies is also enough to cause Satou to become unhinged.
Her moral compass is entirely centered on love, or better said, what she believes love is. In her mind, anything done for the sake of love is good and anything that betrays love is bad. She also gets mad whenever someone uses the word "love" to describe a feeling she doesn't recognize as such. Despite having already killed a person before the events of the series, murder is always Satou's last option when she has to deal with people who may ruin her life with Shio. After all, now she knows how hard it is to safely get rid of bodies and clean bloodstains. It's much more efficient to bribe and manipulate them into doing what she wants. Many of her behavioral symptoms are tied to the Yandere trope.
Ideally, Satou wishes to build a castle for her and Shio to live in and spend their days together forever, away from the corrupted and filthy outside world.
At a young age, both of Satou's parents passed away due to an unknown cause, leaving Satou in custody of her aunt. Her aunt taught her about different kinds of love throughout her childhood, raising Satou with deranged ethics regarding love consequently. Her childhood sparked her curiosity in experiencing love for herself, leading to her many relationships with males and earning her a reputation for being "easy to get". At some point, Satou began working at a café and met Shouko, who became her friend due to their common interest in finding a real relationship and accompanied Satou in her endeavors with guys.
One rainy day, Satou encountered an artist who allowed her to come to his apartment to dry off because she did not want to go home to her aunt. Satou offered to repay him in exchange for the shelter and modeled for a painting. From then on, she regularly visited his apartment until she met Shio.
After meeting Shio, Satou stopped hanging out with guys and began taking on more jobs in order to earn enough money to support Shio and herself.
- The name Satō (さとう) means "Sugar".
- While Satou is obsessively in love with Shio Kōbe, Satou is never shown to have sexual feelings or intentions towards her. This may be influenced by Satou's desire to preserve Shio's purity and innocence.
- Shio describes Satou's heart as an "empty, but not broken" jar when they initially met.
- Satou makes all of Shio's clothes with the exception of her blue long-sleeved school uniform.