The first season of Chainsaw Man brought about its fair share of tragic deaths, one of which was Aki's close friend Himeno. The devil hunter was Aki Hayakawa's first partner in Public Safety, and she taught him everything he knows about the trade. Following her death, he struggled with intense grief, but facing the Ghost Devil helped him process Himeno's passing.
The penultimate episode of Season 1 reintroduced the Ghost Devil, with it under a new contract with Akane Sawatari. This particular foe was difficult for Aki to face, and at first, it looked as though the Ghost had him bested. However, a gesture from the devil saw the odds of the brawl flipped on its head. Here's why this might have happened.
Himeno Fused With the Ghost Devil
During Special Division 4's first face-off with Akane and other allies of the Gun Devil, the group struggled to fight Katana Man, so the fatally wounded Himeno exchanged her entire body for the help of the Ghost Devil. In doing this, she merged with the devil-- hence why there was no corpse left behind after the fight. This is also implied in Episode 9's ending, in which Himeno's body merges with the Ghost.
The imagery of outstretched hands is also symbolic of Himeno's character. She used the Ghost Devil's right hand as part of her contract, so whenever she was using the power, her arm reached out. As well as this, the Ghost devil has hundreds of hands instead of legs, presumably from other people who have merged with the devil. This symbolism is also suggestive of Himeno's future with her contracted devil.
Himeno's Arm Reached Out to Aki
Just as it seemed Aki was going to be strangled to death by the Ghost Devil, it suddenly put the devil hunter down. One of its arms reached toward him, holding something in its palm, and when the hand unraveled, it held a cigarette labeled "easy revenge." This small gesture was obviously from Himeno, with the arm reached out being her own. When she fused with the Ghost Devil, Himeno became one of its many arms, so the one that reached out to Aki did so out of Himeno's resolve.
This sign from his mentor reminded Aki of Himeno, and more importantly, the things she told him about the Ghost Devil. Its eyes are sewn shut, so it sees fear rather than movement. Previously, Aki was scared when fighting the monster, but after being reminded of this by Himeno, his fear subsided and the Ghost Devil could no longer track his movement. This is why Aki was able to calmly maneuver onto the Ghost Devil and decapitate it with a blow from his sword, killing the devil.
Himeno was able to briefly take control of the Ghost Devil from Akane at just the right time, not only saving Aki but giving him the resolve to defeat the enemy. Shortly after these events, Aki decided to smoke the cigarette given to him -- his last physical reminder of Himeno -- showing he was finally able to say goodbye to his good friend and mentor.