Yosuke Endo is a detective in the Gifu Prefecture police force, investigating bizarre murders where victims are splattered onto walls. His parents died in a monster attack during his childhood, leaving him orphaned. Fifteen years before current events, he befriended a married alien couple expecting a child. After learning they were targeted by a xenophobic organization within the police, he attempted to warn them. They were murdered eight days later, and Endo adopted their infant daughter, raising her as Rena Endo. To protect her, he accepted a promotion from his complicit superior, remaining on the force as her safeguard.

The Science Special Search Party (SSSP) halts his murder investigation. Frustrated, Endo later witnesses Shinjiro Hayata using an Ultraman Suit during a highway rescue and recognizes the SSSP crest, rejecting Shinjiro’s claim as Ultraman. This fuels his resolve to uncover the truth about aliens and the SSSP. With his partner Kurata, he reexamines a murder scene, discovering the victim was a fan of idol Rena Sayama. They chase a four-eyed alien suspect, confirming alien existence when it is struck by a car and reveals its face.

Endo’s protective focus remains Rena. He rushes to her hospital after an alien battle and sets traps with Kurata for the serial killer, unaware the SSSP monitors them. When Bemular warns him of Rena’s danger, he intervenes at her concert to apprehend Alien Igaru. After Rena discovers her alien heritage and flees, Endo retrieves her from the SSSP with Ide’s discretion. He confronts Shiro Maki—a repentant member of the organization that killed Rena’s parents—who poisoned the police chief responsible. Endo introduces Maki to Rena, facilitating her reconciliation and decision to join the SSSP. Endo supports this, believing it safer than her acting alone. He later aids SSSP strategic meetings against threats like the Zetton Core.

In the "ULTRAMAN SUIT ANOTHER UNIVERSE" Tiga arc, Endo assists investigations into ritualistic murders tied to a black-robed trio. With Kurata, he identifies a victim as serial killer Kuriki, though Moroboshi halts their probe. They encounter gravitational anomalies and witness battles between a tricolored Ultraman Suit wearer and a jet-black Ultraman.

Endo respects the original Ultraman, who saved him in childhood, fueling his initial disdain for Shinjiro’s "fake" identity. He evolves from distrustful outsider to collaborative SSSP ally, driven by justice and protecting humans and aliens alike. His partnership with Kurata strengthens, and he considers founding a dedicated alien investigation department. Bemular provides Rena a watch-like protective device, intertwining Endo’s efforts with alien allies to ensure her safety.