The principal characters of Breath of Fire IV are Ryu and his companions, each with their own individual skills and personality traits carry the story forward. Ryu is amnesic young man with the mysterious ability to transform into powerful dragons, with his "other half" being Fou-Lu, the principal antagonist and founder of the Fou Empire centuries ago with similar abilities, yet a much more malevolent personality. While Ryu and Fou-Lu represent two different dichotomies, they were originally one being summoned years ago from the realm of the gods in a botched ceremony, torn apart in the process and scattered throughout time. Aiding Ryu on his quest to confront Fou-Lu are several other heroes including: Nina, who shares her name with the heroine of each previous Breath of Fire title, and the reigning princess of the Kingdom of Windia; Cray, a burly member of the cat-like Woren tribe who wields a large wooden post and holds romantic feelings for Nina's sister, Elina; Ershin, an enigmatic robot-like armor who speaks in third-person, it is later revealed that the armor itself has no name, and "Ershin" speaks in this manner because it is relaying messages from the soul of another Endless being, Deis (to whom the armor refers as "Ershin"), who is sealed inside the metal body; Scias, a lanky mercenary and swordsman from the dog-like Grassrunner clan who lives for money; and Ursula, prideful granddaughter of a military commander with kitsune-like features and skilled with guns.
Primary supporting characters include Yohm, a general from the modern Fou Imperial Army with the ability to summon monsters to his aid who see Fou-Lu as a threat to the world; Rasso, an elitist company commander dispatched to the Eastern Kingdoms to search for Ryu, Fou-Lu's key to obtaining his lost godhood; Yuna, a cruel geneticist and occultist who participates in horrific acts of genetic manipulation with dark magic; and Kahn, a muscle-bound chauvinist and comic-relief villain who constantly encounters Ryu's party. Each of the characters of Breath of Fire IV were designed by Tatsuya Yoshikawa, who drew inspiration from other Breath of Fire titles when designing each of the previously established races, as well as traditional fantasy and Asian folklore. The Ryu and Nina from this title would later make a cameo appearance in SNK vs. Capcom: Card Fighters 2 Expansion Edition.