He wasn't given a sprite in the sound novels nor in the PS3 versions. However, he was given an appearance in the manga. He is a man wearing military uniform, is of Japanese descent and sports a goatee. Despite looking older than Kinzo, he is actually much younger than him.
According to Kinzo, he was a filthy coward (like Kinzo's instructor) trying to forget his fear with violence, and would often reprimand his subordinates for slacking off at the drop of a hat. When he is in a good mood, he would boast about his life stories of being a thug.
He was once a thug, ignoring the rules of society. In World War II, he became a commander of Japanese troops stationed in Rokkenjima.
In Requiem of the Golden Witch, Lieutenant Yamamoto appears in Kinzo Ushiromiya's past, as Kinzo's commanding officer. He had summoned Kinzo and had him interpret for the Salo Republic in English. After learning about the gold the Italians were carrying on their submarine, he tried to negotiate with them so that he was able to have some of the gold for himself. However, when that failed, he commanded an attack on them in an attempt to steal it from them, resulting in a massive firefight. Later in the firefight, Yamamoto took Beatrice Castiglioni hostage, and killed Ensign Angelo with his pistol. After that, Kinzo came to Beatrice's rescue, and Yamamoto, thinking that Kinzo tipped the Italians off, shot at him, grazing his ear. Kinzo then shot Yamamoto in the chest, killing him.