Complex characters don't need to be super complex, just more than a simple one-note cardboard cutout. It's fine to staff the background or secondary supporting cast with those types of characters because most of the time they only exist for minor use in a few scenes or maybe 1 storyline and then will rarely be important again. But for any character that's supposed to have a major stake in the story it should be important that there's enough depth to them to sustain the audience's interest for as long as the story goes on. They don't have to be relatable, just understandable. You don't have to be able to root for them (indeed a well crafted villain shouldn't be easy to root for. They should be bad enough to still be rooted against when the rationale behind their behavior makes some sort of sense) you just have to get the reaction the story requires. And they don't need to be morally gray because making everyone morally gray is rarely a sign of proper complexity since moral grayness is often portrayed as something more along the lines of apathy or indifference when really it should be that a character makes both good and bad decisions while being aware of the fact that they aren't straight-as-an-arrow good nor are they puppy-kicking-and-baby-seal-clubbing bad.
My issue here is that her character screams "plot device" more than character. She exists to likely be a jerk but also become interested in the male MC because she forgets about him from their younger days (or maybe because she does remember him and possibly ends up thinking her cruel behavior was justified because it made him better himself and fitting in and being 'normal' like everyone else is super important since this is a Japanese high school story) but then also to easily lose because her past as a bully/queen bee and possibly any continuing behaviors like that which she still exhibits make her an easy rejection for the MC compared to the nice, sweet, awkward-but-kindly female MC. She can't likely become a serious threat or evolve much as a character because it will threaten the dynamic of the two leads So she'll be stuck as a paper thin jerk/bully stereotype for as long as it's convenient for her to be the enemy or a weight on the male MC's sanity.
I'm making a lot of assumptions, yes. But I've also read enough manga to know that the chances of this story breaking out of the industry norms are slim. It's why I look at something like "A Bouquet for an Ugly Girl" with a lot more positivity because it is the rare story that avoids a lot of the typical cliches of the genre (the male MC is nice and popular but he's not a perfect bishonen pretty boy. He's ultimately a relatively average guy who's kind of handsome and nice. Tabata never has to change anything about herself except to gain confidence and we don't see her upgrade from ignored loser to person with friends by changing her hairstyle, ditching her glasses, losing weight, or anything else. She simply grows as a person. the main secondary girl who is sort of the rival (her name starts with an "U" and is quite long) is kind of a secret jerk at first, but she quickly realizes how terrible her behavior would be if she pushed it and genuinely gets over her hang-ups to become friends with Tabata and the growing circle of included students. Genuinely it's just refreshing to have the characters all have their own hangups and issues and characterizations that are more than the single loudest note in their individual proverbial symphonies.
"The Fragrant Flower Blooms With Dignity" has a similar advantage. Largely because a major focus of the story is "everyone is more than they appear to be and nobody is defined by the single biggest trait others identify them with" And the series benefits from that because I don't hate anyone in the cast given that each of them has more going for them than their initial impression (delinquent jerk, sullen moody weirdo, cheerful hyperactive goof-off, ice queen, reserved "proper" girl, etc)
