Matsukichi, called Matsu, began as a lone thief. His path changed when the Five Leaves' leader, Yaichi, intervened during a theft gone wrong, saving him from pursuers. This act left Matsukichi owing Yaichi a life debt, compelling his continued loyalty and service to the group.

Operating as a spy for the Five Leaves, Matsukichi gathered intelligence on targets. To support his reconnaissance with legitimate cover and independent income, he ran a small business crafting women's metal hair ornaments. This trade later sparked misunderstanding; after Masanosuke saved his life, Matsukichi presented Masanosuke's sister with an expensive hair ornament purely as gratitude. Others misinterpreted the valuable gift as inappropriate affection.

Typically calm, collected, and reserved, Matsukichi often appeared emotionally detached, especially near Umezou. He maintained a quiet presence, usually keeping distance from other members save Yaichi. Yet beneath this stoicism lay determination and occasional recklessness; once set on a goal, he pursued it relentlessly, regardless of risk.

His hidden past surfaced unexpectedly: he discovered he had a wife and at least one son, the circumstances and timing unspecified. When his son was injured, Matsukichi stole money to support them. Injured during the break-in, he was captured and imprisoned. Confinement stripped away his usual reserve, exposing emotional vulnerability as his past was revealed, demonstrating deep personal commitment beneath his reserved exterior.

His relationship with Masanosuke deepened significantly after the life-saving act, creating a secondary debt of gratitude that complicated his primary loyalty to Yaichi. Matsukichi's development centered on challenges to his emotional control, particularly during imprisonment, uncovering core motivations of family obligation and personal loyalty driving his actions within and beyond the Five Leaves.