Alex Kandley Stewart is a young man of Mexican origin who, alongside his two childhood friends, escaped poverty in his home country and joined the Annex Project. His primary motivation for undertaking the dangerous mission to Mars was to obtain American citizenship and pursue his dream of becoming a professional baseball player. Following the death of his mother, Alex moved to his father's hometown in northern Mexico at the age of seven. There, he first met Marcos Garcia and later Sheila Levitt. At twelve, he worked as a paper boy for meager wages. By the age of seventeen, he and Marcos were living as illegal immigrants in Sunny Pool, California, struggling for food and a chance at the American Dream. His goal is to find Sheila, who had gone to the United States earlier, and achieve his dream of becoming a major leaguer.

Alex's personality is defined by his deep love for baseball, having greatly admired major league players ever since attending a game with his parents as a child. He throws with his right hand and bats with his left. He is characterized as a caring individual, particularly towards his friends, and possesses a lively, competitive spirit, especially in his rivalry with Marcos. This bond is strong enough that he stands up to Marcos in a fight, yet remains a loving friend. He is driven and principled, strongly opposing Marcos's decision to join a street gang, as it would mean abandoning their true purpose of finding Sheila and becoming professional baseball players. He chooses to leave his friend rather than compromise on their shared dream. Despite this, his loyalty ultimately prevails as he comes to Marcos's rescue when the gang decides to kill him for not following through with their plan.

In the story, Alex is one of the participants in the Annex I mission. After losing hope and seeing no other opportunities, he and Marcos decide to head to U-NASA to undergo the surgery that would allow them to go to Mars. Upon arrival, they are surprised to find their friend Sheila alive, having previously believed her to be dead when they saw her mother's face in the newspapers. A notable moment in his early involvement is a confrontation in a back alley where he and Marcos argue over food, leading to a brief scuffle that neither has the energy to finish. After Marcos appears to have sided with local gangsters, a disillusioned Alex leaves, declaring he does not want to see his friend again. He later returns to save Marcos, using a thrown rock with great precision to disarm the gangsters, and the two fight together against them.

Following his M.O. Operation, Alex receives the powers and abilities of the harpy eagle. This transformation grants him the ability to fly and makes his eyesight eight times stronger than that of a normal human. His upper-body muscles become incredibly strong, and thanks to this enhanced gripping strength combined with his natural human body, which is well-suited for throwing, he is able to throw objects at speeds that make their impact comparable to a bullet. With his exceptional eyesight, he can launch projectiles with pinpoint accuracy from impossible distances, often outside the enemy's detection range. In combat, he utilizes specialized multi-gadget balls, which can be bladed or explosive, making him perfectly suited for ranged attacks against Terraformars. He can also increase the damage of his attacks by pitching the balls to Marcos Garcia, who bats them with his specialized staff, causing them to fly even faster.