In the Final Fantasy VII Crisis Core (PSP) game, Cloud is a rookie member of the Shin-Ra (mega-body energy company) security squad, in other words the commercial and technically modern army of that day. Cloud at that time is an innocent young man who yearns to be a SOLDIER, whose mission is the peace of Shin-Ra, but because of his few abilities he fails. Luckily, in a mission carried out by Zack with Tseng in a factory, he is closely linked with Zack, who becomes his only and best friend. In Final Fantasy VII Cloud he mentions that he had a person so close that he could consider her as his brother, although he does not mention who, it can be assumed that he was referring to Zack. Zack protects Cloud from that moment, and even Cloud helps Zack in the battle he has with Sephiroth in the Mako reactor to the point of defeating Sephiroth but with the help of Zack.

P.S He is depression in Ult