At the beginning of Chainsaw Man Part 2, a new main character for the manga was introduced. Asa Mitaka was an average, quiet high schooler who hated all devils as a result of her parents being killed by one. However, after being killed by the Justice Devil, Asa was saved by the War Devil who saw potential in her as a vessel. Yoru resurrected and inhabited Asa’s corpse, turning her into the War fiend, but left half of Asa’s brain intact to keep her alive.
While Denji is no longer the central antagonist of the series, there are consistent callbacks to Part 1, reminding the audience of how much the ex-devil hunter experienced and resurrecting common themes of the initial story. Asa acts as a reminder of all the tragic women in Denji’s past, but even more so, her relationship with Yoru is symbolic of one of CSM’s core themes.
Chainsaw Man’s Part 1 Focused on the Coexistence of Happiness 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.
This also reflects in other characters, such as Aki Hayakawa. He lost his entire family at the hands of the Gun Devil, but if this hadn’t happened, he’d have never befriended Denji, Power and Himeno and learned the significance of found family. Even the latter’s death showed Aki and the audience to accept the tragedy of death and move into happiness — that grief is essential.
Yoru and Asa Represent the Morning and Night

Part 2’s focus shifts to Asa and Yoru, whose names translate to morning and night. After becoming Asa’s inhabitant, Yoru realized that without her host, her plan couldn’t be fulfilled. The War Devil is hunting Chainsaw Man, as she wants to force him to revive the Nuclear Weapons Devil, making humanity’s fear of war stronger. Denji attends Asa’s school, and while Yoru doesn’t know he’s the one she’s seeking, the devil knows that Chainsaw Man is a student, so without her vessel, getting close to her target is difficult. Hence, morning cannot survive without night, or light cannot exist without darkness.
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.
CSM consistently depicts feelings of aimlessness and coming to terms with personal struggles while exploring the human experience. Characters are consistently overcoming trauma as best possible and finding a place for themselves in life, even when it seems insignificant. Even if getting there hurts, through accepting the dark, light can always be found.