A Horse with No Name

1. Namae no nai Uma (A Horse with No Name)
The story (the prequel to MARS) takes place after Rei and Tatsuya enter High School. Tatsuya, who at the time has a crush on Kira, dislikes Rei and picks fights with him. After the climatic motorcycle chase between Rei and a reckless gang of bikers, Tatsuya and Rei begin a new friendship. Other main characters of MARS like Harumi are also presented.

The other two chapters are earlier works of the author non-pertaining to MARS:

2. Nemuru Raion (Sleeping Lion)
Tells of the strange relationship between Isomu, a guy who pretends to not have any fear, and Ayame, a girl with particular behaviours.

3. 1 Karatto no Kajitsu
Tells of Nina, who has some family problems, and Rui, her classmate who is interested in a close friend of the hers. But what if he's interested in Nina?

(Source: Anime News Network, edited)