Mars - Volume 1

Rei took notice on Kira since they met in the park and Rei asked Kira for direction to the hospital, and finds out that they are in the same class.  Kira didn't like Rei since he's the playboy type that smokes and gets in trouble in school.  But Kira changed her mind after Rei complimented on her painting of the mother and the child and saved her from their teacher who sexually harrassed Kira.  When Kira asked Rei to model for her and also to give him a painting of the mother and the child, Rei agreed, and in turn he said that he'll protect her from anything. 

Tatsuya, Rei's best friend and has a cruh on Kira since junior high, sees Rei and Kira liking each other, gives up on Kira but warns Rei not to hurt her.  Neither Rei nor Kira admits that they are attracted to each other.  The other girls in the schools get jealous because Rei is giving Kira so much attention.  Harumi, Rei's former fling, harrassed and threatened Kira to not "show off" with Rei anymore and tells her that she might die sooner than she thinks. 

Rei starts to model for Kira but he is always restless when modeling.  Rei tears up Kurasawa's drawing of him when he finds out that he has been drawing him secretly without Rei's permission.  Kurasawa's a senior who has won drawing contests and people expect him to do well as an artist.  Rei further offends Kurasawa when he comments that his painting had no soul in it.  When Kurasawa saw Kira's drawing of the mother and the child, he stole it and copied the drawing into a painting and submitted for a contest and won first place. 

Rei saw the painting in the paper and went to Kurasawa's classroom and threatened him.  As a revenge, Kurasawa got some thugs to beat up on Rei, but Rei ended up huting the thug.  On Kira's end, Rei's most recent ex-girlfriend threatened Kira not to talk to Rei again, and Kira refuses.  Harumi sees that and saves Kira from the girls. 

Worried about Rei since he hasn't returned to school even after his suspension was over, Kira skips school and goes to his house to see him.  They started to hang out and had a lot of fun together.  On two occasions when the two saw someone jumps building to kill themselves (one was a stranger and the other Kurosawa), Rei acted really weird, the first time he even had problem breathing and Kira had to give him CPR.  Kira finds out that Rei had a younger twin brother who died, named Sei.  The volume ends with Rei going back to see his doctor and the doctor asking Rei if he still feels that he caused Sei's death.

Mars - Volume 3

Rei takes Kira to the racetrack to watch him ride his motorcycle.  Rei is called to ride in the race of his life, the Suzuka 8-endurance, but Kira's mom would not allow her to attend the race.  As Kira roots for Rei from miles away, Rei races to the finishline first, but only to be disqualified because of a regulation.  After the race, Kira takes Rei to see her finished painting of "Mars" at the art exhibit, and the painting won first price.  The volume ends with a girl looking at the painting with tears in her eyes.  

Rei's ex-girlfriend Shiori from junior high, goes out her way to see Rei at his school.  She's a beautiful girl and always gets what she wants - and she wants Rei back.  Kira, being the timid girl she is, allowed Shiori to get close to Rei and even suggested that Rei should spend time with her.  When Rei tells Shiori not to bother him anymore, Shiori walks into traffic attempting to kill herself.  Rei saved her from the traffic and it intensified Shiori's obssession with him.  Kira finds out that Shioro was really Sei's girlfriend, but since Sei was too timid to make any move on Shioro, Rei stole Shiori from Sei.  Takemura, Rei's old schoolmate, believes that that's why Sei killed himself.  Shiori and Kira talked and Shiori said that Kira reminds her of Sei, and when Rei told her to stop sounding like Sei, Kira was bothered because she does not want Rei to want her only because she reminds him of his lost twin brother. 

On the anniversary of Sei's death, Kira goes with Rei to visit his grave.  Rei tells her that his mother used to tell the twins that their father is a bad person, and Rei has just started to question why she said that, and that he's sick of counseling.  Shiori shows up at the grave and tells them that Sei killed himself because she told him to go away so that Rei would be with her.  Getting upset, Shiori runs to the roof of their school, where Sei killed himself, and Rei tells Shiori that Sei didn't kill himself because of her - the real reason is that Sei found out that their father isn't really their father at all.

When Kira finds Rei sick at home, she ends up staying over to take care of him.  While there, Rei tells Kira that he found out that he and Sei are results of their mother's affair with a guy.  He told Sei about it because he knew Sei wouldn't be able to handle it, and he hated Sei because he didn't like Sei and wanted him to go away.  The reason why he had a nervous breakdown was because he realized how violent he was as a person. 

At school, Rei shows changes by actually going to school.  I guess Rei and Kira at this point are in a gray area because Kira said that they had a platonic relationship but she didn't mind.  Takemura shows up and tells Rei that Shiori is waiting for him to see her and tell her how he really feels toward her.  Rei told Kira to wait for him and as he went to see Shiori.  When Rei sees Shiori, he tells her that he can't be just friends with a girl, and he's found someone who he cannot live without.  Kira waits (foolishly) for Rei and thank goodness he actually went back to her.

At the art show where Mars was being shown, a man shows up wanting to buy the painting.  Kira refuses.  What happens next is that Kira refuses to get too intimate with Rei.  When Harumi finds out, she tells Kira that she should have sex with Rei or she will lose him.  Harumi tells Kira that she has slept with Tatsuya. 

Lastly, a new student who is a guy but is as pretty as a girl, shows up at school.  The girls at school are all interested in him, Masao, but he reveals that he already likes Rei.