One Merseian

The Day Of Their Return does not need any Merseians on-stage. First, all of its action, except for one brief reminiscence by Chunderban Desai, occurs on the surface of the planet Aeneas in Sector Alpha Crucis on the opposite side of the Terran Empire from the Merseian Roidhunate. Secondly, the only agent of Merseia on Aeneas during the course of this novel is Aycharaych, a Chereionite. 

However, Desai's reminiscence is about his dinner with Uldwyr. The latter mentions Starkad, Jihannath, Talwin, the recent insurrection in Sector Alpha Crucis and the Domain of Ythri and thus references the contents of the preceding four novels in the Technic History. We have now seen a succession of four powerful Merseians:

Morruchan

Brechdan

Ydwyr

Uldwyr

The humbler Tachwyr, whom Flandry met on Merseia, has by now appeared twice. He will become Flandry's opposite number, rising in parallel with him through the ranks, so that, when Flandry is an Admiral, Tachwyr will have become the latest successor of Brechdan. Future history is also fictional biography.