Nezuko developed Bipolar Disorder due to the mental and physical stress that was put in her body. Bipolar Disorder is a mood disorder characterized by manic or depressive symptoms, interspersed by periods of normal mood (McCormick et al., 2015).
Tanjiro — Imposter Syndrome & Self-Doubt
After the events of the Mugen Train Arc, we see Tanjiro grappling with self-doubt and imposter syndrome as he grieves the loss of Rengoku.
Tanjiro is suffering from trauma and nightmares from the Mugen Train Mission, and despite his determination he is slowly succumbing to the grief and despair that is suffocating him. He tries to hide his misery from everybody but once the truth comes to light, there will be no hiding place left.
He often claims that there is no tomorrow for him, because of the risk of uncertainty to life, that comes with the profession. Dealing with a constant state of anxiety, Zenitsu often tends to avoid vulnerable situations.
What is Nezuko's age? When the series starts Nezuko is 12-years-old since she is one year younger than her brother. Between the first chapter and the end of chapter 5, time has gone and two years have passed. While stuck physically at age 12 because of the whole demon thing, Nezuko is 14 chronologically.
According to the creator of Demon Slayer, Koyoharu Gotouge, Nezuko's biological age is 12. This means that while she has been alive for 14 years, her body has only aged to the point of a 12-year-old.
So, as a result, it can be concluded that Zenitsu does not really have a split personality disorder, but instead sleepwalks, or rather, fights. Zenitsu Agatsuma is a fan-favorite character of Demon Slayer due to his endearing nature and intense fighting skills.
Nezuko Kamado
Zenitsu also protects Nezuko multiple times when she is in danger, evident during Zenitsu's first meeting with Inosuke Hashibira and the battle against Enmu in the train. Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
Demon Slayer: Analyzing the concluding chapters to ascertain Nezuko's love interest. Nezuko marries Zenitsu Agatsuma in the Demon Slayer series. Let's understand how we arrived at this conclusion.
As Tanjiro pushed past the threshold of pain during this fight, he almost managed to kill Daki. The fuming rage that Tanjiro felt from seeing Daki destroy human lives helped him push past his limits while also making him bleed through his eyes in anger, resulting in his eye colors changing to bloodshot red.
Nezuko and Bipolar Disorder Nezuko's caring personality changed after her family was slaughtered by the home invasion. Nezuko developed Bipolar Disorder due to the mental and physical stress that was put in her body.
Nezuko Has Suppressed Her Consciousness
Frequently, she is pictured carrying or soothing smaller siblings, clearly taking on a maternal role at a young age. However, after being transformed into a demon by Muzan Kibutsuji, the Demon King, she quickly regressed in mental age and maturity.
After the death of Tsutako, Giyu tried to tell others that she was killed by a Demon, however, he was labeled as mentally ill and was sent away to one of his relatives, a doctor. Giyu ran away during the journey there and almost died on a mountain, but he was rescued and became apprenticed under Sakonji Urokodaki.
At the end of the chapter, there is a picture of Inosuke giving acorns to Aoi while she smiles. It is confirmed in the Volume 23 extras that Inosuke and Aoi did eventually end up together and that they have two great-grandsons, one of which is Aoba.
Zenitsu's debt is estimated to be around 600,000 yen in modern currency. This massive amount of debt likely came from his 7 ex-girlfriends. Despite all of them being beautiful, they treated Zenitsu like a slave, tricked him as well as robbing him of his money.
Zenitsu is also shown to be something of a wannabe womanizer; he tends to hit on girls who he thinks are cute and ask them to marry him, much to their annoyance.
Zenitsu has some combination of RBD, narcolepsy, sleepwalking, and lucid dreaming that allows him to behave as if he were awake while asleep.
Believing he was losing his master's favor, Kaigaku began to despise Zenitsu for being merely a weak and timid man who earned his master's favor out of dumb luck. Thus, he started to bully Zenitsu constantly.
Nezuko shrinks to fit inside that wooden backpack or to evade demon slayer swords, and nothing else. So, Nezuko's size can change either way to reflect her emotional state and instinctual needs,. Her size-changing isn't merely a visual gag or a fun party trick.
It's implied that the characters from the original series have all since died and have been reincarnated in modern day Tokyo in the final chapter of the series, but two key characters confirm that Nezuko Kamado and Zenitsu Agatsuma eventually did have a family together as their great grandchildren, Yoshiteru and Toko ...
She doesn't “turn into an adult” She just changes her physical appearance to be that of an older woman. She very much has the mind of a young child and is only 14 chronologically.