|“||Son of a bitch! You want some? Get out here and fight fair!||”|
—Tsuyoshi, "The Ruined Phantom Planet"
Beast King Golion
Tsuyoshi is one of the five students of the Fuji Space School who returned to Earth after a space flight in 1999, only to find it in ruins after a great war. Captured by the Galra Empire, the pilots escaped to the planet Altea. Tsuyoshi's strength allowed him to bend the bars of their cell window to provide a route of escape. Once through the window, he revealed his fear of heights, though he was able to put it aside. 
He later meets a girl who calls herself Lisa, and learns she is from Earth and that survivors from Earth were captured and enslaved by Galra. He mentioned he had a sister, whose fate he did not know yet. 
Super Robot Wars W
Tsuyoshi, along with the Golion team, appear in the Japan-exclusive crossover game Super Robot Wars W. The team debut in Chapter 7, fighting alongside with Nadesico on Mars against both the Galra and Jovians. The Golion team explains that they were captives of the Galra and escaped to a ruined Planet Altea and became the pilots of Golion.
After seeing the ruined planet, the team decides to travel to Earth to warn them of the Galra Empire's destruction. On Earth, the team protect the GGG Headquarters while losing Shirogane in the process.
- His surname of "Seidou" is the Japanese word for Bronze/Copper.
- In Girl of the Land of Evil, Tsuyoshi states that he had a younger sister whose fate is unknown.
- Tsuyoshi is fond of Children.