Saitou Toshihisa is the longtime best friend and unrequited love interest of Nagai Atsushi. He possesses a notably youthful appearance, often described as having a baby face, and exhibits a naive, childish demeanor. His marriage ends abruptly on the wedding night when his wife abandons him to elope with her coworker, an event occurring immediately after the ceremony where Nagai served as best man.

Following this abandonment, Saitou becomes emotionally vulnerable and increasingly reliant on Nagai for comfort and support. He begins to display romantic interest in Nagai, initiating advances that complicate Nagai's existing entanglement with Honda Kenzou, a wedding guest who had a brief encounter with Nagai. This shift creates a conflicted dynamic among the three characters.

The narrative confirms that Saitou harbored genuine romantic feelings for Nagai all along, a revelation emerging as he processes his failed marriage. His background prior to these events centers solely on his friendship with Nagai.