Dr. Yoshinaga researches the Gibia virus and develops its vaccine, collecting venomous stingers from Gibia monsters with help from Sensui Kanzaki and Kenroku Sanada. He has long blue hair, wears glasses, and dresses in a light blue shirt, dark blue pants, and a white coat. After encountering Sensui and Kenroku—time travelers from the Edo period—he accepts their origin story, reasoning that Gibia monsters' existence makes time travel equally plausible. He explains how Gibia spread by stinging humans and transmitting the virus. During a Gibia attack on their camp amid a blackout, he directs Kathleen Funada to locate her mother and recruits Kenroku to support the defense team. To repel the monsters, he devises a strategy to burn tents and create light. He admires Sensui’s combat skills and Kenroku’s rapid mastery of modern weapons like grenades.

His background involves extraterrestrial origins from a planet where consuming the blood of a god named Soma granted powers but transformed incompatible beings into monsters. While creating artificial Soma blood, his fiancée Meteora accidentally ingested it and became a monster. Fleeing planetary chaos with Meteora and failed artificial blood samples in a cryogenic capsule, he aimed to develop a cure. The capsule shattered during Earth’s atmospheric entry, scattering its contents. Meteora and the samples landed first, triggering the global Gibia outbreak upon impact. His ongoing research seeks a virus cure to save Meteora.