Megumi Fushiguro's story in Jujutsu Kaisen has a bittersweet edge; he has always been dedicated to protecting his stepsister, Tsumiki. The moment she enters concord with a cursed coma was when all of Megumi's decisions as a sorcerer began to revolve around her.Tsumiki is Megumi's main reason for fighting and remains the major influencing force on Megumi while battling the evil denizens of the jujutsu world. If not for Tsumiki, there is a good chance that Megumi would never have set foot inside Jujutsu High School.Tsumiki as seen in Megumi's memory (Image Source: MAPPA Studio)Fans are now questioning if Tsumiki really ever existed outside of being the reason Megumi fights. Tsumiki exists almost entirely through Megumi's memories in Jujutsu Kaisen. She doesn't have a voice, she doesn't have any agency, she isn't present in any way, or form other than motivating Megumi. She only exists in his memories and raises the question if Megumi's biggest motivation for fighting came from his idealization of her rather than actual events.Tsumiki’s being a part of Jujutsu Kaisen is tough to swallow for fans because she isn’t really her own character outside of being a flashback filtered through Megumi’s lens. Most of the other supporting cast have been given plenty of time to shine at some point throughout the series, but Tsumiki is never truly able to leave a lasting impression on viewers.It's pretty clear by the way she has written out of the story that Tsumiki doesn't mean anything to anyone else in the narrative. Megumi is the only family member who really cares about her. Gojo had technically been watching over both kids after their dad left, but really, he was just interested in what kind of ability Megumi could become as a user of the Ten Shadows.The Idealized Sister Who Never Was in Jujutsu KaisenTsumiki in a coma state (Image Source: MAPPA Studio)To Megumi, Tsumiki was Everything. Tsumiki was Megumi's reason for being and his light in the darkness. Megumi thinks of Tsumiki as perfect: Dedicated, genuine, kind, and bright. The comparison to Yuji is accurate because Yuji gives Megumi his guiding light through life.While Megumi views both Tsumiki and Yuji as perfect, they both create flags of concern because the little bits we see of the association between Megumi and Tsumiki show a complex relationship with both commonalities and differences that Megumi chooses to ignore or forget.In this flashback scene, Tsumiki really lets Megumi have it when she gets upset that he doesn't care about her concern for him fighting. She throws a milk carton at him in frustration, which is not the more saint-like version of Tsumiki we see later on.A Character Reduced to a Plot Device in Jujutsu KaisenYorozu reincarnates inside Tsumiki Fushiguro (Image Source: VIZ)It seems like in the Jujutsu Kaisen fan community, many them think Tsumiki was primarily used by the author, Gege Akutami, as a plot tool for developing the character of Megumi and not as a standalone character with an independent arc.In a recent statement, Gege even said they wish they had used her character more during the Culling Games Arc, thus confirming a lot of readers' beliefs that Tsumiki was originally written to act as a vehicle. Basically, you find out that Tsumiki was "dead" before you even read about her in the first place. From the time she woke up after the Culling Game, it was really throughout the possession of Yorozu (a past sorcerer).Megumi’s whole reason for protecting Tsumiki goes down the drain when he realizes she is gone. Because Megumi couldn’t see that she was possessed, it shows just how badly he didn’t know his sister.Megumi's Shattered FoundationWhen Megumi's entire drive was about saving someone who barely mattered in the story, it became a matter of when, not if, his ultimate breakdown would come to be.In Jujutsu Kaisen, we learn that Megumi's purpose was always so tenuous in the fact that it was formed out of an idealized memory rather than an actual connection with the person. He wouldn't even know if the person who replaced his sister was really her or not, proving that a huge portion of who "Tsumiki" was still existed only in Megumi's mind.This realization forces readers to reconsider Megumi's entire character arc in Jujutsu Kaisen. Megumi's full potential as a Sorcerer never came out because he didn't want anything that really mattered enough to him personally, from a human standpoint. There is no way for Megumi, who only fought for a memory of an idealistic past, to possess the same kind of selfish resolve as Gojo, Sukuna, and other powerful Sorcerers.So Megumi's ultimate weakness has nothing to do with his technique or mindset. Instead, it is that the original reason he was fighting may have never really existed at all.