Nana Komatsu a.k.a. "Hachi" is one of the two main characters of the manga by Ai Yazawa known as Nana. She is one of the two Nanas in which the series revolves around with Nana Osaki a.k.a. Nana O being the other one. The apartment number known as "707" highlights the significance of the two Nanas. Nana is known as "seven" in Japanese. Of all of the characters, Nana K or Hachi is perhaps the most flawed character in the series. However, these personality flaws make Hachi into one of the most interesting characters.
Hachi's story begins around the same time as Nana O's. Hachi is moving out to Tokyo to be with her boyfriend Shoji Endo who is currently going to art school. On the train, Hachi cannot find any seats. She finds the only empty seat. Hachi ends up sitting next to Nana O. They become acquainted with each other. Soon, they do not really see each other for some time. While living under the same roof, Hachi's childish attitude and demands get to Shoji. He tells Hachi to find her own place.
Soon, Hachi and Nana O run into each other when renting an apartment. The two of them are at odds with each other because they both want to rent the same apartment. However, the two of them come into an agreement to be roommates. This way, the two of them can split the monthly rent. From then, their friendship starts as they are doing various oddjobs.
It is revealed that Hachi is a huge fan of the band known as "Trapnest." She happens to be a fan of the leader of Trapnest, Takumi Ichinose. However, Hachi is suspicious about why Nana feels uneasy around Trapnest. Soon, Hachi wins tickets to see Trapnest perform at her hometown. It is revealed that the guitarist of Trapnest named Ren Honjo is Nana O's ex-lover. At the same time, Hachi becomes part of the reformation of "BLAST" as their mascot.
Since the concert, Hachi gets pulled into a web of lies, deceit, and skeletons in the closet that already entangles most of the members of BLAST and Trapnest. However, Hachi's own personal nature causes her to willingly get dragged into the web. This is an example on Hachi's personality. In a sense, she has the personality of a puppy that wants affection. That is stacked on top of the fact that Shoji cheated on Hachi with his co-worker named Sachiko. Eventually, Hachi gets involved in a love triangle involving Nobu of BLAST and Takumi of Trapnest.
Hachi's feelings towards Nobu are genuine and at the same time her feelings towards Takumi are the result of being a major fangirl. While Hachi starts to lean towards Nobu, it is then revealed that she is pregnant. The child ends up belonging to Takumi. For financial reasons, Hachi ends up with Takumi because he has the finances to support a child while Nobu does not. On top of that, Takumi also does it out of duty.
The flawed personality of Hachi becomes a crucial driving force for the story. It creates all sorts of obstacles and conflicts for most of the characters. She fades away from Nana. Nobu ends up feeling all sorts of emotional pain. Yasu ends up worrying about how it all affects BLAST. Reira feels her affections toward Takumi go unreturned. What makes Hachi more interesting is that she serves as the comic relief. As the story progresses, she does add humor but loses her role as the comic relief. She ends up being the housewife to Takumi. Her role becomes more serious as she learns about Nana's past and does whatever she can to help. Of all of the characters, Hachi does represent improvement while retaining a good portion of her personality flaws. Perhaps she is the most human of them all. She has transitioned from a person that could not help herself if her life depended on it to a person that could somewhat hold her own in the time of need. Currently, it is unknown what the future for Hachi is in store due to the manga currently being on hiatus.