Zephiel is the heir to the Bern throne, and the son of Queen Hellene. Zephiel is a very gifted child, and it is obvious that he will become a fine ruler of Bern. However, his spiteful and paranoid father, King Desmond, tries to stop him from taking the throne, wanting his favored daughter Guinivere's future husband to rule instead. Despite this, Zephiel and his half-sister, Guinivere, love each other dearly, but Desmond also wants to bring an end to their contact, starting with the order of killing a baby fox that Zephiel captured and gave to Guinivere as a pet.
During the game, Zephiel is almost murdered by Jaffar and Nino. However, Nino overhears his prayer asking for his family to live in happiness and that he can be accepted by his father. She relates with him due to her own struggles to be accepted by Sonia, whom she believes to be her mother. She thus refuses to kill him. Jaffar, who feels compassion towards Nino due to her saving him in the chapter "Four-Fanged Offense", disregards his orders and the two of them attempt to flee, leaving the prince only unconscious. Eliwood, Hector, and Lyn ultimately rescue Zephiel from Black Fang troops led by Ursula, who attempted to finish off the unconscious prince.
Zephiel, now king of Bern, is the main villain of Fire Emblem: The Binding Blade. In this title, Zephiel's sanity is slipping away from him, and he seeks to rid the land of Elibe of humanity by allying himself with dragons, believing they would not repeat humanity's mistakes.
Many years after the events of Fire Emblem: The Blazing Blade, Desmond attempted a second assassination by handing his son a goblet of poisoned wine; Zephiel took it under the belief that his father had finally accepted him. Zephiel fell ill from the poison, but did not die. However, with the help of his close ally Murdock, he stayed in hiding and had a false funeral arranged. When Desmond stepped up to the casket to view Zephiel's body at the funeral, Zephiel, now in a lust for revenge, rose and stabbed his father to death.
A few days later, Zephiel used the Fire Emblem to awaken the Demon Dragon Idunn. Shortly afterwards, the dragon Jahn sends an illusion of himself to confront Zephiel about his motives, and when it is revealed that they have similar goals, the two started working together. Five years after this, Zephiel ordered his army to take over the continent of Elibe.
The nomadic nation of Sacae was attacked by a force led by Brunnya and the first to fall. Next was the frigid nation of Ilia, with the Bern forces led by Murdock. Then he personally came to the nation of Lycia after Narcian's attack on the Lycia Alliance and spoke to the fatally wounded marquess of Ostia, Hector, who would die shortly after Roy and the Pheraen Army came to save him.
Zephiel later forged an alliance with the corrupted nobles of the Kingdom of Etruria, Roartz, and Arcardo, and caused a rebellion there; he personally went to an attack himself with Flaer, Narcian, and begrudged Perceval, and knocked out Cecilia after she lost to him in battle. When he left, and the Lycian army was noticed, Narcian left Flaer to command, and he was killed by Roy. Later, in Chapter 22, Zephiel sends Brunnya with Idunn to the Dragon Temple to complete his plan and later faces Roy, who manages to defeat him.
He can be unlocked as a playable character in the trial maps after the game is completed 7 times.