Isidro is a young runaway thief who becomes a member of Guts traveling party. He is a short boy with spiky, dark red or brown hair, and his physical appearance and mannerisms have been noted to resemble those of a monkey. By the time of the Fantasia arc, he is approximately fourteen years old.

A boy from a village, Isidro ran away from home with the ambition of becoming a great warrior. His dream is to one day be known as the greatest mercenary or swordsman of his time. This aspiration is fueled by the legendary tales of a formidable captain known as the Hundred Man Slayer, who once fought in the original Band of the Hawk. Isidro is unaware that the object of his admiration, Guts, is that very same legendary figure.

Isidro is headstrong, cocky, and often boasts about his abilities, a trait he shares with his close friend, the elf Puck. Despite his bravado, he has a good heart and is shown to care for his companions, even if he bickers with them constantly. He has a particularly strained relationship with the young witch Schierke, who frequently calls him a monkey and finds his intellect lacking. However, he has proven willing to put himself in danger to protect her and other female companions. As a teenage boy, he is also easily flustered around women and has displayed a mischievous, curious interest in matters of romance and the opposite sex.

Isidro first encountered Guts when he was surrounded by Kushan scouts. Awestruck by Guts' incredible swordsmanship as he dispatched the enemies with ease, Isidro decided to follow him. While he insists he is only following Guts to steal his techniques and refuses to call him master, he deeply looks up to the swordsman as a father figure and role model.

Within the group, Isidro serves as a student and a junior combatant. Guts does not teach him directly by showing him his own moves, believing that each person must develop their own style based on their unique physical attributes. Instead, Guts provides encouragement and the basic principles of survival. Over time, Isidro develops a fighting style that uses his small size and natural agility to his advantage. He wields a cutlass given to him by an old soldier named Morgan and a magical salamander dagger, which can produce flames, given to him by the witch Flora. He is also exceptionally skilled at throwing rocks with great accuracy and later receives a bag of small bombs from Guts. While often providing comic relief alongside Puck, Isidro has proven himself a capable fighter, able to hold his own against experienced human opponents and even kill trolls and other monstrous creatures.

His relationship with Guts is the core of his development. Although Guts rarely shows affection openly, he has demonstrated faith in Isidro abilities, encouraging him to find his own path as a warrior. Guts has found in Isidro an apprentice, to whom he offers words of wisdom and guidance. Isidro is extremely close with Puck, who often sits on his head and sees himself as a mentor. Puck attempts to teach him the so-called Elf Dimension Style, a form of combat that relies on trickery and misdirection, with limited success but great comedic effect.

Throughout the journey, Isidro matures from a brash and inexperienced boy into a more confident and resourceful young warrior. He learns to use his natural talents and the tools at his disposal, steadily contributing more to the party in combat situations. His dream of becoming the greatest swordsman remains, but his understanding of what that truly means continues to grow as he travels with Guts and the others.