He and Light's other, unnamed friend from high school would often discuss Kira with Light. The two of them gradually grow to become Kira supporters and believe that the deaths of criminals will be for the betterment of society.
Yamamato eventually joins the Task Force within the year after Light died. As revealed in chapter 108, he and Matsuda share a friendship. According to Matsuda, they've gone drinking together, apparently more often than Yamamoto would like. Yamamoto also makes an appearance in the One-Shot Special.