A brand-new manga main character was introduced at the start of Chainsaw Man Part 2. Asa Mitaka was a typical, reserved high schooler who detested devils because one killed her parents. However, Asa was saved by the War Devil after being killed by the Justice Devil because he recognized her potential as a vessel. Yoru preserved half of Asa's brain to keep her alive while resurrecting and inhabiting her corpse, transforming her into the War fiend.
While Denji is no longer the series' main antagonist, there are frequent references to Part 1, reminding the audience of how much the ex-demon hunter went through and resurrecting common themes from the first story. Asa serves as a reminder of all the tragic women in Denji's past, but her relationship with Yoru represents one of CSM's central themes.
Part 1 of Chainsaw Man's Was Concerned With the Coexistence of Joy and Tragedy
One of the core themes symbolized throughout the primary part of the series was how the day cannot exist without the night, or light cannot exist without darkness. This allegory essentially suggests that there is no good without the existence of evil or no happiness without periods of sadness. People must accept the bad or malicious parts of life to appreciate the good things that happen. Without this, individuals grow up detached from reality or ignorant, so this acceptance is essential to humanity.
As the main character, Denji is a good example of this metaphor. He grew up impoverished and abandoned, and this is exactly what makes him so pleased to have the bare minimum. He has a roof over his head, good meals throughout the day and eventually, companionship, and because he experienced such a steady stream of trauma, he can recognize the improvements in his life. Even more so, Denji craves intimacy and connection, but throughout Part 1, every attempt to achieve this ended in disaster. Now, he is the guardian of the young Nayuta, and because he experienced a life devoid of family and affection, he knows exactly how to nurture this child and make her a better person. Without his trauma, Denji wouldn't be able to recognize when goodness entered his life or how to be virtuous to those around him.
Other characters, like Aki Hayakawa, also show this. He lost every member of his family to the Gun Devil, but if that hadn't happened, he would never have made friends with Denji, Power, and Himeno or understood the importance of "found family." Even the latter's passing demonstrated to Aki and the audience that grief is necessary in order to move past the tragedy of death and into happiness.
The Morning and Night Are Represented by Yoru and Asa
The focus of Part 2 shifts to Asa and Yoru, whose names mean morning and night, respectively. Yoru discovered that her plan couldn't be carried out without Asa after moving in with him. In order to revive the Nuclear Weapons Devil and increase people's fear of war, the War Devil is pursuing Chainsaw Man. Denji goes to Asa's school, and even though Yoru is unaware that he is the person she is after, the devil is aware that Chainsaw Man is a student, making it challenging for her to approach her target without a vessel. So, just as light cannot exist without darkness, neither can morning survive without night.
Similarly, after Yoru took over Asa's body and saved her life, she threatened that should she ever oppose her, the War Devil would kill her. This means that Asa quite literally cannot live without Yoru, further implying the symbolism of Chainsaw Man's Part 1. As individuals, Yoru, the night, and Asa, the morning, must coexist to survive, much like happiness and tragedy or evil and good.
While examining the human experience, CSM frequently depicts feelings of aimlessness and coming to terms with personal struggles. Even when it seems inconsequential, characters consistently overcome trauma and find a place for themselves in the world. Even though getting there hurts, light can always be found by accepting the dark.