Seiko is a genuinely cheerful person, but does have a side of her that is impulsive and jealous, and she lacks a tolerance for Haruka's stupidity. Seiko's impulsive nature causes her to act without thinking, notably when she forces a kiss on Ōtani in front of his friends before ever having talked to him. She later does this again to Kazuki Kohori. Seiko gets extremely jealous of Risa when she realizes Ōtani is wearing a sweatband Risa gave him and tries to get rid of it. Even though Seiko is usually cheerful and upbeat, she does get upset when it comes to her gender identity, like when she believes Ōtani thinks she's gross for being transgender and starts crying.
With her petite stature and fashionable mind, Seiko is arguably the most feminine character in the series. Seiko's hair is typically worn down with a side-high tail, but will occasionally wear a normal ponytail or beanie hat.
In both the anime and manga, Seiko has shoulder length blonde hair and reddish-brown eyes. During school in the anime, Seiko wears a uniform consisting of a white dress shirt with a frilly collar and sleeves, blue knee-high socks with pink stripes and lace, a large red ribbon around her neck, and pink shoes with a light pink bow. During warmer weather, she has on a peach sweater vest with red stripes and a star, and is the only female character to wear a red skirt. In the winter, Seiko wears a peach cardigan with red stripes and a star, and replaces her socks with blue knee-high socks that have white stripes and no lace. Seiko got in trouble on the first day of school for wearing normal clothes instead of her uniform because the outfit was too cute not to wear.
After Seiko's voice change (because she's a trans girl), she decided to go back to her dead name, Seishiro, and start dressing like a boy. She came to school with short hair, wearing a hoodie, khaki shorts, knee-high socks with a stripe, and tennis shoes. With Risa's encouragement, Seiko realized that she shouldn't give up on who she really is simply because her voice changed. The next day, she showed up looking like the old Seiko again (in a wig now) and explained that her voice had actually sounded deeper due to a cold.
- Kazuki Kohori (previous): It is unknown when Seiko and Kohori met, started dating, or when Kohori found out that Seiko was transgender. Kohori did break up with Seiko after finding out that she is transgender.
- Atsushi Ōtani (previous): Seiko developed an immediate crush on Ōtani after he saved her from a dog on his way home from school. Seiko found Ōtani the next afternoon at school and kissed him. It is unclear if they actually started dating after that, but they stopped seeing each other after Ōtani found out Seiko was transgender. Despite being really uncomfortable regarding the situation since Seiko had not disclosed this to him prior, Ōtani still complimented Seiko and continued to be her friend.