Chereionite

Dominic Flandry visits the planet Talwin at least three times and it is there that he has his first sight of Aycharaych. Because of his involvement in the events leading to the joint Terran-Merseian research station on Talwin, Commander Flandry is temporarily assigned to that planet in the hope that his special knowledge will be useful for intelligence gathering purposes. Noticing a member of an unfamiliar species on the Merseian side, he enquires and is told something of Chereion:

an incredibly ancient ace with secret powers;

a formerly highly civilized race, possibly regarding Merseia as merely a means to its own ends.

Since we later learn that Aycharaych is the last Chereionite, it must have been he that Flandry spotted on Talwin. 

When, in "Honorable Enemies," Aline Chang-Lei, the Lady Marr of Syrtis, asks Flandry what he knows of Aycharaych, adding that Flandry has roved around more than most, Flandry's answer must be informed by what he was told on Talwin. He tells Aline that:

Chereionites are privileged within the Roidhunate;

he knows of no one who has seen Chereion or even knows where it is;

Aycharaych is a very successful field agent.

A partly true but also misleading picture.