Alfonso San Valiante is the crown prince of the kingdom of Valiante. On his twentieth birthday, his mother, Queen Esmeralda, gives him a golden pendant she had owned throughout her life. Embedded in the pendant is the symbol of Garo, a red gem in the shape of a triangle contained within a golden circle. Alfonso is noted to have been a fairly frail and sickly child, however, he managed to grow up as a strong, healthy, and formidable swordsman. Despite his status as a prince, he holds a down-to-earth attitude and treats all people kindly. Alfonso is also courageous and intensely committed to his royal duties, willing to go to any length to protect his kingdom and people. When his father's adviser Mendoza usurps the crown, Alfonso managed to escape capture, going about as a fugitive to seek out the Knight of Light to correct the wrongs. In his travels, he finds a mentor in Rafael Banderas, also known as Gaia the Knight of Defense, who agrees to teach him the ways of the Makai Knights.
Rafael reveals to Alfonso that Queen Esmeralda is actually the biological sister of Anna, the Makai Priestess who was burned at the stake 17 years before the present, thereby making him related by blood to the lineage of the Golden Knight and the current Garo's cousin.
During the battle with Mendoza, Alfonso confiscates the Garo sword and armor from León after the latter is defeated as the Lost Soul Beast. The armor accepts Alfonso as its rightful bearer and he uses it to defeat the powerful Horror Mendoza summons. He refuses to return the armor to León, finding his cousin to be an insult to the legends of the Golden Knight and Makai Knights as a whole.