Eygon is a warrior who is charged with guarding Irina. He’s a terse individual, and isn’t very friendly. However, he’ll do his job. If you release Irina from her cell (in the Undead Settlement), Eygon will agree to let her travel to the Firelink Shrine. Should you harm Irina, he’ll turn aggressive on you.
If you purchase any Dark Miracles from Irina, she will fall to darkness, and Eygon will kidnap her. You can encounter him just outside the Iudex Gundyr bonfire when this occurs. If you do not anger Eygon, his last appearance is as a White Phantom for the Dragonslayer Armour battle, after which his corpse appears in Irina’s original cell in the Undead Settlement.