Seamus Pegg was once an adventurer, famed throughout Teyvat. When he came to Mondstadt, he started anew with the Church of Favonius and later rose to the position of Seneschal. He fell in love with Frederica Gunnhildr of the prestigious Gunnhildr Clan; they married and had two daughters, Jean and Barbara. The two eventually separated and Barbara stayed with him.
Seamus first appears in Chapter 1 of the manga, attending the Ludi Harpastum festivities at the Dawn Winery. When Il Dottore and Krupp demand an audience with him, Seamus shows himself. The three of them, as well as Diluc and Kaeya, head to the Dawn Winery's meeting room to talk about the Fatui's desire to recruit people from Mondstadt.
While Seamus' name is not directly mentioned in the Archon Quests, his position as Seneschal is. When attempting to retrieve the Holy Lyre der Himmel, Gotelinde tells the Traveler that a signed document from the Seneschal is one of the items required to borrow the artifact, and has optional dialogue where she mentions that Seamus is away. After resolving the Stormterror incident, Seamus is mentioned when the Traveler goes to return the Holy Lyre der Himmel; Barbara states that he has been pushing for its return.
- A seneschal is a senior position filled by a court appointment within a royal, ducal, or noble household during the Middle Ages and early Modern period — historically a steward or majordomo of a medieval great house.
- Seamus Pegg was originally named "Seamus" in the manga and "Seamus Pegg" in the Closed Beta Test versions of Jean and Barbara's character stories. For Version 1.0, he was renamed to "Simon Page," but it was reverted to "Seamus Pegg" in Version 1.4.
- In Korean, he was renamed accordingly from 시몬 페이지 Simon Peiji to 시무스・페그 Simuseu Pegeu, but this change was not reflected in Barbara's Hangout Event.
- His name remains unchanged in Chinese (西蒙・佩奇 Xīméng Pèiqí), Japanese (サイモン・ペッチ Saimon Pecchi), and French (Seamus Page).
- In Japanese, his name is inconsistently localized as サイモン・ペイジ Saimon Peiji in Barbara's Hangout Event. It is unknown which one is correct.