Masamune Kido commands Border, the organization defending Mikado City against interdimensional Neighbors. His pivotal actions position him centrally within major conflicts, notably as the primary antagonist of the Black Trigger Capture Arc.

Kido possesses a lightly tanned complexion, short slicked-back ebony hair, and slate grey eyes. A prominent scar runs down the left side of his face, a remnant from Border's early days; expert medical reconstruction failed to repair the damaged eyebrow. He consistently wears formal suits befitting his rank. Historical depictions show him with longer, unscarred features, often dressed in a navy blue cardigan, dress shirt, and loafers, sometimes captured grinning—a stark contrast to his current stern expression.

As a founding member of "Old Border" with Masafumi Shinoda, Takumi Rindō, and Yūgo Kuga, Kido initially championed peaceful coexistence with the Neighborhood. The deaths of comrades during Neighbor conflicts radically altered his stance, replacing optimism with a hardline ideology that fueled Border's recruitment. Despite this reputation, he permitted exceptions, allowing Neighbors like Yūma Kuga (Yūgo's son) and Hyuse to join Border under specific terms.

Kido enforces strict adherence to Border's rules, expressing stern condemnation for violators like Osamu Mikumo, though he later offered Osamu measured encouragement. His leadership blends pragmatism and ruthlessness—authorizing lethal force against perceived threats while approving financial incentives for agents undergoing arduous testing. Deep-seated hatred for Neighbors born from trauma coexists with nuanced judgment; he tolerates Tamakoma Branch's association with Aristeran refugees and eventually approved Hyuse's enrollment. Jin Yūichi once suggested Kido's actions conceal a "true objective" beyond stated ideologies, a claim persuasive enough to halt his pursuit of Yūma's Black Trigger.

Key relationships define his complexities:
- **Yūma Kuga**: Kido initially ordered Jin to eliminate Yūma as a hostile Neighbor. Learning Yūma was Yūgo's son, he deployed A-rank teams to capture his Black Trigger but prohibited lethal force. Following Jin's negotiation—exchanging the Black Trigger Fūjin for Yūma's safety—Kido permitted enlistment. He later promised protection contingent on Yūma obeying rules and allowing limited Black Trigger use during invasions.
- **Osamu Mikumo**: Kido used Osamu as a scapegoat post-invasion but later defended him against critics. He acknowledged Osamu's growth, offering encouragement before rank battles and accepting Osamu's warning against repeating the scapegoating incident with Yūma or Chika.
- **Former Colleagues**: Complex ties persist with fellow Old Border founders. Kido referred to Shinoda as a "mischievous brat," reflecting longstanding friction. Director Rindō noted Kido's rigid ideology strategically aided Border's recruitment despite underlying complexities.

Kido's role shapes major events:
- **Black Trigger Capture Arc**: He ordered Yūma's elimination and later the seizure of his Black Trigger. After Jin brokered the Fūjin deal, Kido sanctioned Yūma's Border membership.
- **Border Enlistment**: Kido questioned Rindō's support for Tamakoma-2's goal to rescue Chika's brother as unrealistic. He later interrogated Yūma about impending invasions, securing critical intelligence from Yūma's Replica after promising safety and proving sincerity.
- **Large-Scale Invasion**: Kido authorized Yūma's Black Trigger use but restricted him to designated danger zones to prevent public panic.
- **Ongoing Governance**: He oversees strategic decisions like Away Mission stipends and Hyuse's enrollment, demonstrating operational ruthlessness balanced with organizational foresight.

Kido values his family, enjoys old movies, and prefers black coffee. His scar remains a visible reminder of Border's formative struggles.