What mattered the most was remembered the least.
358/2 Days, alike CoM, is often considered as a spinoff. It was released for the DS in 2009 and was later remade into a three hour long mini-movie for the Kingdom Hearts 1.5 PS3 release in 2013. The game covers the missing year, from when Roxas enters the Organization until his departure, and thus heavily features him as well as Axel and Xion (and the rest of the Organization). Kairi does not appear at all in this game - at least not directly - but in the interest of being complete, I'll cover it.
Upon his "birth", Sora's nobody is found by Xemnas and given a name - Roxas. He is then drafted into the Organization as the thirteenth member, and is highly prized for his ability to use a Keyblade, as it makes collecting hearts for Kingdom Hearts that much more easier. At first, Roxas can barely form a sentence, but as his first week progresses he develops a strong bond with Axel, who watches out for him, and regularly takes him to the clock tower in Twilight Town to share ice cream.
Sometime later, around the time Axel leaves for Castle Oblivion, Roxas is introduced to the fourteenth member of the Organization, who turns out to be a girl around his age, Xion. Like him, Xion can also wield a Keyblade, but her ability to do so is sporadic. In fact, when Roxas struggles, Xion does well with it, and vice versa. Xion is revealed to be a copy, modelled after Sora's memories of Kairi, but she is incomplete, and to give up her memories that belong to Sora means to give up her existence entirely. Disgruntled at the lack of answers, Roxas eventually leaves the Organization and Axel behind, fights and defeats Xion, and blunders into a fight against Riku. This is around the point that he and DiZ capture him, and place him into the virutal Twilight Town in order to reintergrate his memories with Sora's.