Not much is known about Seoras' history except that he has been Queen Sgiach's guardian for centuries.
Seoras MacUallis first appears at the entrance of the Isle of Skye when James Stark, Aphrodite LaFont, and Darius are trying to gain entry to help fix a shattered High Priestess, Zoey Redbird. At this first meeting, he appears hostile and dismissive of them, but changes once Stark makes it clear that he is part of the MacUallis Clan, and fetches Queen Sgiach.
To help Stark to get to the Otherworld, Seoras is tasked with keeping him on the brink of death and does this by using a ceremony formed by old magick and cuts him over and over again until his task is completed. The magick in the ritual means that he is finely attuned to Stark and knows what every mark means for him. Whenever he is interrupted he appears to be in a trace like state.
When in the Otherworld, he guides Stark in the right direction. The first time he appears in the Otherworld, it is in his physical form handing over the Claymore, whilst telling Stark of the responsibilities of becoming a Guardian. The main way he guides Stark however comes in the form of a golden piece of material the warrior wears on his wrist in the physical realm.
It was mentioned that Queen Sgiach was in mourning over the death of Seoras.