Jujutsu Kaisen author Gege Akutami has become infamous since the manga's conclusion for not developing a lot of characters, especially Megumi Fushiguro, because of his innate talent as a sorcerer prodigy. However, there is a major example of how the mangaka ruined a character through his writing, and that is Nobara Kugisaki.
Many people have pointed out how Nobara's character was mishandled in Jujutsu Kaisen and how she was one of Akutami's many casualties, especially regarding her role as one of Satoru Gojo's students, alongside Megumi and Yuji Itadori. It is also worth pointing out that she has a lot of natural talent as a sorcerer that went nowhere throughout the manga.
Disclaimer: This article contains spoilers from the manga/anime and reflects the opinions of the writer.
Explaining how Gege Akutami's writing ruined Nobara Kugisaki's potential in Jujutsu Kaisen

Nobara Kugisaki has often been highlighted by the fandom, alongside Megumi Fushiguro, as one of the most wasted characters by author Gege Akutami. It's because her past, motivations, and development as a sorcerer were never fully explored, often handled in a hand-fisted manner, and her abilities were mostly ruined by Akutami's writing.
Nobara's Cursed Technique, Resonance, is quite practical and effective when considering the element of range and how it allows her to punch above her weight, as evidenced by the damage she caused to Mahito in the Shibuya Incident arc and Ryomen Sukuna at the tail end of the Shinjuku Showdown arc. Considering that she was capable of doing this without much experience and training as a sorcerer, she could have done a lot more.
However, Akutami never focuses on developing Nobara as a character and as a sorcerer, which is a mistake when considering she was trained in the main team of the sides, with Satoru Gojo as her teacher and Megumi and Yuji Itadori as teammates. Therefore, the story never gives her the time to grow stronger, which is even more frustrating given her eventual return at the end of the manga.
The many problems of Nobara's character

While Megumi Fushiguro is often highlighted as the most wasted Jujutsu Kaisen character by Akutami, Nobara can compete in that area. As Mahito seemingly killed Nobara during the Shibuya Incident arc, she didn't have the time to develop since she was removed from most of the second half of the manga. Nobara returns to help Yuji against Sukuna at the eleventh hour in the final battle, which was widely criticized by fans.
Moreover, considering how she was a part of the main quartet with Yuji, Gojo, and Megumi, she is not connected to any meaningful plot point in the story, with the possible argument that her biggest contribution was dying in Shibuya. This is further emphasized by the extra chapter in the final volume, where her reunion with her mother, a rather minimal element of her character, was treated with indifference by many readers, highlighting how much Akutami wasted her.
Final thoughts
Jujutsu Kaisen has a lot of issues as a story, especially after its conclusion, and it goes without saying that characters such as Megumi Fushiguro and Nobara Kugisaki were handled quite poorly. Moreover, Nobara being a fan-favorite is even more telling when considering how she was removed from the story and how her potential as a sorcerer was never explored.
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