My Hero Academia has had a lot of characters changing their costumes throughout the story, such as the protagonist, Izuku "Deku" Midoriya, but the example of Shoto Todoroki is quite prominent. Not only because his initial design had him covering half of his body in ice, but also due to the fact it highlights his character development throughout the series.
Shoto's growth and the entirety of the Todoroki family subplot in My Hero Academia are quite celebrated, ranging from Endeavor's redemption to the twist of Dabi's revelation. Within that context, the changes the youngest Todoroki child goes through in terms of his design play are a great example of how much he grew in the story.
Disclaimer: This article contains spoilers from the manga/anime and reflects the opinions of the writer.
How Shoto's costume changes highlight his development in My Hero Academia

When Shoto Todoroki is first introduced in the series, he is seen with half of his body covered in ice, thus representing the peak of his hatred toward Endeavor and the abuse of his family that the latter committed. Moreover, this is the character at his most resentful and arrogant, which is also highlighted when he calls out Izuku "Deku" Midoriya as inferior to him and says that he is going to defeat him in the Sports Festival to prove it.
There is a strong argument to be made that author Kohei Horikoshi abandoned the idea of Shoto covered in ice because he didn't think it worked, but, be that as it may, this ends up resulting in a thematic development. His upcoming design is rather simple but also holds some similarities to Endeavor's costume when it comes to the color scheme and some of the line work.
Furthermore, by the time of the final war arc, when he is dealing with Dabi and having a somewhat improved relationship with Endeavor, Shoto appears with a costume that looks similar to his father's but holds his own identity. This is something that is fully confirmed in the eight-year time skip, where he is no longer seen as Endeavor's son.
Shoto's character and the Todoroki family subplot

A common opinion in the My Hero Academia community is that the Todoroki family subplot is widely regarded as one of the strongest elements of the series, with a lot of people praising Horikoshi's writing of characters such as Shoto, Endeavor, and Dabi. Moreover, the fandom has praised the author for tackling the topic in such a good manner and doing so while paying respect to several points of view, shown through how each Todoroki child reacts to their father's attempts at redemption.
There is also the element of Dabi's revelation as the first Todoroki child, Toya, and his burning hatred for Endeavor, to the point he recorded and released a video to destroy the latter's hero image. Furthermore, the conclusion of this subplot presents Endeavor leading with the consequences of his actions while Shoto manages to break away from this trauma and live a good life.
On the other hand, Dabi didn't get the dangerous and destructive death he always wanted since he aimed to create as much damage as possible to spite his father. He arguably has a rather grim death in My Hero Academia, dying connected to a machine with his body mostly burnt.
Final thoughts
Shoto's design changes throughout My Hero Academia prove how much he grew in the series, where he manages to move past his hatred of Endeavor and also find his own identity as a hero. It is one of the most complex and celebrated subplots in the series, which is something that plays a role in the success of the franchise from a writing perspective.
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