Jo Katsuragi is a young man with dark hair who serves as one of the primary protagonists in the story. He is defined by his reticent nature and his absolutely deadly combat abilities, which he displays whether fighting unarmed, wearing a Light Warrior suit, or piloting a Gladiator mech. A very quiet and introverted individual, Jo speaks very little, and his silence is a notable impediment to understanding his inner world. He has a unique and involuntary physical reaction after piloting a Gladiator, as tears stream down his face following combat; he regards this phenomenon as almost like a side-effect.

Jo's background is one of hardship. He grew up among the oppressed Levinas class, joining the elite Phantom military force at a very young age. Within Phantom, he was recognized as the most dangerous and deadly member, a reputation that caused his previous partners to fear him or be killed trying to keep up with his abilities. It was during his time in Phantom that he met Jin Tsurasawa. Jo developed a deep sense of loyalty towards Jin because Jin was the first person who was not afraid of him and stuck with him as a partner. When Jin decided to desert Phantom and take the mysterious girl Sana Nobuto with them, Jo followed without hesitation.

Despite his deadly efficiency and cold-blooded demeanor, Jo is a highly sensitive person. This sensitivity is evidenced by his tears after piloting, as he is capable of perceiving the implanted consciousness or emotions within the Gladiator machines. While he fiercely protects Jin, his role in the story also reveals a hidden care for Sana, even though she initially views him as scary. A significant moment in his development occurs when Sana discovers that it was Jo, not the more openly affectionate Jin, who physically saved her life during their escape. This revelation helps build a bond between the silent warrior and the young girl, suggesting that one of his motivations for deserting Phantom was to provide Jin and Sana with a purpose, moving beyond simply being the perfect soldier for an organization. Jo is highly skilled in mecha piloting, unarmed combat, and the use of military exoskeletons known as Light Warrior suits.