My Hero Academia owes a great debt to the X-Men. Both focus on schools that train children born with extraordinary gifts how to harness and control their abilities, often for the good of mankind. It isn't difficult to imagine the students of U.A. High School fighting alongside those of the Xavier Institute. In many respects, the manga makes attempts to link up to its American comic counterpart -- but where the two differ, things get interesting.
Tsuya Asui is one of the most popular characters in My Hero Academia. Originally one of the fold, her popularity skyrocketed as she appeared, which resulted in the addition of entire episodes to the anime that expand her minor moments from the manga, only so fans can have more of her. She's also a counterpart to the X-Men villain Toad, one of the most reviled characters in the franchise.
Toad and Tsuya share several obvious similarities: Both possess frog-like traits, allowing them to leap large distances and flick incredibly long tongues. Tsuya's abilities go further than that, however, as she can also secrete mucus from her skin that allows her to walk up walls, vomit out her stomach to clean it, and creates toxins. She later learns how to camouflage herself by altering her coloration and body temperature.
But arguably an even more striking similarity is how both don't look like ordinary people. When he first appeared, Toad resembled a relatively ugly person, but over time became more frog-like, growing green skin and pointed ears. Tsuyu possesses oversize hands and feet, buggy eyes, and almost snout-like lips that are often inflexible; she also tends to ribbit.
So on the surface, they're very similar, but the real differences emerge when you look at how their characters develop.
People often remember Toad as a henchman, merely another member of Magento's Brotherhood of Evil Mutants. He is often part of the bigger gang, and, while he sometimes does take charge, he's usually depicted as pursuing petty, lesser goals.
Many writers have attempted to give Toad more personality. He would question Magneto's motives or tactics, and in later stories he develops a genuinely sweet relationship with the mutant Husk. He learns how to light his tongue on fire; he shows emotional vulnerability. After all, Toad is a mutant rejected by all of society, he has little else but bitterness and resentment.
However, none of those changes have evolved Toad past his niche: He's someone's lackey. Whenever he is written with any context, it is usually framed as Toad, the henchman, having a day in the limelight. He isn't a character in his own right, and, often, none of it changes that he is the henchman.
Tsuyu, Class 1-A
Contrast that with Tsuyu, who's introduced as one of the many students of Class 1-A, the best of the best. So right from the start, Tsuyu is no henchman. She's an equal to the main character, Midoriya. Tsuyu also is one of the smartest students in the class, shown to use her powers in inventive ways, and, whenever the story centers on her, she's given the initiative in every action she does.
These are important elements of difference. Tsuyu is never defined as "the henchman" like Toad is. She isn't even defined by her power-set. She is just another student in Class 1-A, the same as everyone else. Because of that, she is written as a character in her own right, and not merely filling a role.
But there is one other key difference, too. Tsuyu's arc is clear: become a hero. Toad's arc is ... well, he's a bad guy. Whenever Tsuyu does something, it's to push her further along to becoming a hero. What is Toad's arc? Toad often exists as a means for another character to succeed. Magneto uses him as a henchman. The Brotherhood hangs with him to complete some group objective. But what Toad wants is almost inconsequential.
However, it isn't only about goals. Tsuyu and Toad are different in that her emotional needs are presented as important while his ... aren't.
Toad is a pathetic character who laments being pathetic. He is ostracized by society and works for whoever will accept him. Tsuyu starts in a similar place. Her childhood is spent being bullied by her classmates due to her inhuman appearance. It's only in U.A. that she finds anyone who accepts her. And she loves that.
When Katsuki Bakugo is captured by the League of Villains, Tsuyu warns her friends it might be best to let the heroes save Bakugo rather than go out on their own. Naturally, Midoriya and the others don't listen to her. Come the end of the arc, she's in tears because she felt her friends no longer valued her opinion, and while she understood why they went, she felt momentarily worthless to them. That moment is framed as sincere and emotional.
Toad deciding to split from the Brotherhood to do his own thing is ... a henchman doing his thing.
The only time his emotional needs are given any weight is in regard to his relationship with Husk, which is played as a small side-story rather than a focus of the greater narrative. It starts up in Avengers vs. X-Men, after all. That was done basically to give Toad something to do. It plays no role in the larger story.
The sad reality of truth is this: Tsuyu is Toad done right, because Tsuyu matters.