Fire Emblem 6: Another Story - Champion's Sword

Taking place on the fictional continent of Elibe and staring Roy, the son of Fire Emblem protagonist Eliwood. Roy leads the League of Lycia's army against the forces of the militant country of Bern shortly after his father falls ill.

The story begins when King Zephiel, ruler of the kingdom of Bern, finishes the brutal conquest of Ilia and Sacae and sets his sights on Lycia. With the war coming to his own country, Roy is sent home to lead Pherae's army to oppose Bern, but soon after he meets Guinevere, the princess of Bern and Zephiel's younger sister, who has escaped from her homeland to search for a way to stop the imminent war between Bern and Lycia.

Upon the death of marquess Hector, Roy carries on the Lycian League and makes many promises: to protect Princess Guinevere and Hector's beloved daughter, the mage Lilina, and most importantly, to save the entire continent from Zephiel's mysterious thirst for world domination.

But this story's hero is Al, son of a Bernian blacksmith. After turning 15, he left his home with his father's sword to go on an adventure in this continent bound for war.

Side Story of Fire Emblem - The Binding Blade (Gameboy Advance Videogame).