Donov Mayer is the Regent of Romdo, sometimes also referred to as the Representative. He is often seen with an Entourage-type AutoReiv by his side, as well as the Collective. He himself never speaks, using the Collective as his "voice" instead.
He is also Re-L Mayer's grandfather.
Proxy One appeared to Donov some time before the beginning of the series, handing over the rulership of Romdo to him. It is possible that this was to allow Ergo the freedom to travel to Mosk in order to be with Monad Proxy. Presumably, Donov was the one who ordered the invasion of Mosk.
Donov is killed by Proxy One in the final episodes of the show.
Speculation: How long ago was it that Donov Mayer was given control of Romdo? In the flashback scenes in episodes 19 and 20, it is shown that Donov mayer is a still middle-aged man, not young, but neither is he advanced in age. Given that the domes, especially Romdo, have very advanced medical technology, it is possible that Donov Mayer is over a hundred years old, possibly even hundreds of years. Given the state of decay at Mosk it looks quite likely that many hundreds of years have past since the war with Mosk, waged by Donov Mayer. Some evidence of this is the fact that Daedalus Yumeno is put in charge of guarding Monad Proxy after Monad had been in possession of Romdo for some time; at least this is implied by Daedalus in his conversation with Re-L concerning the Proxies. The only complication to this are the immigrants, who seem to be recent arrivals in Romdo. This also raises questions of how travel between the two cities was accomplished by both the army of Romdo, and the immigrants from Mosk .
He doesn't speak a single word as reagent, including during his death. It is repeatedly implied that he genuinely loves Re-L, though his actions are not always consistent with this.