Whoever is assigned this role would be the Voice Actor for Keigi Hakaisha.

Keigi is Dark Isemiovrei, a Dark Ultra who is the father of Ultraman Vylcan and Ultrawoman Anastasia. He is also the former leader of the criminal organization known as the Typhonian Project and was corrupted into his current, dark form by the Reaprea‘s Lord Particles. 

His corrupted form generally has a tactical, ruthless, egotistical, and overall brooding or serious composure. However, this is a facade used to hide a more unpredictable and cruel version of his nature wherein he is prone to short bursts of maniacal laughter and may quickly devolve into a much more violent persona.

The reason this role is not being paired with another character, despite Keigi’s relatively low number of appearances, is because Keigi is the most complicated voice on the series: Whoever voices him will basically need to be able to transition from using a typical, deep villain voice to using a voice resembling being on the verge of a maniacal laugh reminiscent of the Joker, and there’s very likely still going to need to be room for much expression beyond that.