Grathan Gryn is the Sub-commander of King Annex's Zaboom Army, assigned to eliminate Princess Romina of Ladorio. He initially tracks the Xenos 5 spaceship to the Solar System before concentrating on the princess as his primary target. Grathan enters a reluctant alliance with Hazzard Pascha, Mars's dictatorial commander, though their collaboration remains fraught with mutual distrust and manipulation. Hazzard perceives Grathan as a competitor and frequently undermines his authority, while Grathan harbors resentment toward Hazzard's opportunism.

Hailing from the influential Gryn family—a military dynasty commanding substantial resources and technological advancements on their homeworld—Grathan wields inherited privilege and authority to further his personal ambitions for power. He demonstrates ruthless calculation, readily sacrificing lives to accomplish objectives. His personality reflects strategic precision: he favors intricate planning over brute force, maintains a reserved demeanor that conceals true intentions, and operates through logic rather than emotion.

Grathan employs psychological manipulation as a core tactic, exemplified in his dealings with General Icelander. He traps Icelander and proposes a deceitful exchange: three ninja robots for Princess Romina’s assassination. This manipulation drives Icelander to defect from Xenos 5 to the rival faction C-Terahertz. Later, Grathan offers Icelander a revised agreement, pledging Romina’s safety in return for capturing Xenos 5, while fully intending betrayal. During a critical clash, Grathan fires on Romina’s aircraft and destroys Icelander’s Mantis mecha when Icelander blocks the attack in protest, showcasing Grathan’s willingness to eliminate obstructing allies.

Grathan’s role concludes as he prepares to execute Princess Romina. The ninja robot Tobikage intervenes, annihilating him mid-strike.