Isidro is a young boy who first appears as a runaway thief and aspiring warrior. He joins Guts on his journey, becoming a member of the swordsman's traveling party. His background is that of a village runaway, having left home with the dream of becoming a great mercenary or soldier.

In terms of personality, Isidro is brash, cocky, and often acts impulsively, frequently getting himself into trouble. He dreams of becoming the best mercenary warrior of his time, with his ambitions inspired by the legendary tale of the Hundred Man Slayer, a former captain of the original Band of the Falcon. Unbeknownst to Isidro, this hero is Guts himself, who has never revealed this fact to the boy. Despite his loud and sometimes cynical exterior, Isidro has a good heart and shows genuine care for his companions, even if he often bickers with them. He is particularly close to the elf Puck, with whom he forms a comic duo, often bragging about their abilities and getting into mischief together.

The character's primary motivation is to become the greatest swordsman in the world. He follows Guts initially to steal the warrior's techniques, but over time his goals evolve. Despite his insistence to the contrary, Isidro looks up to Guts as a mentor and father figure, seeking the man's approval and wisdom. His role in the story is that of a young apprentice and a member of Guts' growing group of allies. He provides a contrast to the darker themes of the series with his energetic and humorous presence, while also representing a future generation that Guts is inadvertently training and protecting.

Key relationships shape Isidro's development significantly. His relationship with Guts is central, as the swordsman does not outright teach Isidro his own moves, but rather instructs him to develop his own fighting style based on his unique physical attributes. This advice pushes Isidro to innovate. His friendship with Puck is extremely close, with the elf often acting as a self-proclaimed mentor. Isidro also has a strained but caring relationship with the young witch Schierke, who frequently calls him a monkey for his appearance and behavior. He also forms bonds with other party members like Farnese, Serpico, and later the mermaid Isma, showing a protective side. He receives a magical salamander dagger from the witch Flora, and a cutlass from an old man named Morgan, both vital to his fighting style.

As the story progresses, Isidro undergoes notable development. Starting as a thief with little combat ability beyond throwing rocks, he evolves into a competent and resourceful young warrior. He learns to use his small stature, natural agility, and ambidexterity to his advantage. Under Guts' loose guidance and through real combat experience, he develops his own techniques, most notably a spinning move using his cutlass in conjunction with the salamander dagger. He also learns to use miniature bombs gifted to him by Guts, igniting them with his fire-imbued dagger. He demonstrates the ability to hold his own against experienced opponents and contribute meaningfully in battles, though he still has much to learn.

Isidro's notable abilities include his high agility, quick thinking in battle, and deadly accuracy with thrown projectiles. His primary equipment includes a magically crafted salamander dagger that can generate intense heat and combust enemies, a standard cutlass for quick slashing and piercing, and small explosives. His fighting style is unorthodox, relying on low stances, dual-wielding, and unpredictable tactics to overcome his lack of raw strength. He is shown to be one of the few people Guts has ever trained, and his growth throughout the series is a testament to his determination and the effectiveness of learning through real experience.