A runaway brat turned thief, Isidro dreams of becoming a great swordsman, which made him tag along with Guts. Despite his lack of strength and skill, he utilizes his small size and agility to his advantage, along with various tricks and tools. He has a great ego and plans to become the very best; he initially planned to "observe" Guts and steal his "techniques" to surpass him. But he slowly starts to realize the inconceivable difference in strength between him and Guts, but that there may even be ways for him to become a formidable warrior...

His most important role however, is probably to serve as a comic relief along with Puck.