|First game||Fire Emblem: The Sacred Stones|
Lyon is a gentle person who greatly admires his close friends, Eirika and Ephraim, princess and prince of Renais respectively. His greatest ambition was to be a strong ruler to protect his empire but harbored feelings of uselessness fueled by his weak health, his one-sided feelings for Eirika, and his hidden jealousy of Ephraim.
Aided by Knoll, who was among a team of Dark Magic researchers, he discovered and used a "forbidden" spell that allowed him to see into the future. What he saw was a massive earthquake that would destroy most of Grado. He began to experiment with the Fire Emblem, which was the Sacred Stone of Grado that contained the spirit of the Demon King, Fomortiis.
His extensive experimentation eventually caused it to split the Stone in two halves, one of which was still a Sacred Stone, the other of which was the Dark Stone, which now contained the Demon King's spirit. However, when Lyon tried to use the Dark Stone, he fell under the Demon King's influence and was convinced that he might be able to avert the disaster by reviving the Demon King and harnessing his power.
Lyon first appears in a cutscene in the chapter "A New Journey". Eirika and Ephraim share a flashback in which they talk with Lyon about their ambitions in life, etc. In Chapter 18, depending on the route taken, he is either completely possessed by the Demon King, who imitates his soft persona to convince Eirika to hand over the Renais Sacred Stone, or fakes possession (so as not to reveal the truth to his friends) and takes the Sacred Stone from Ephraim by force before revealing himself. He uses the demonic resurrection spell Naglfar in combat, as well as the demonic spell Fenrir and the leech spell Nosferatu in chapter 17.
In the last chapter, he is killed and dies in the arms of his friends, then serves as the final sacrifice needed to revive the Demon King. Despite his efforts, the earthquake he foresaw happens anyway and is seen by people as the divine punishment for Grado.