It is not known if Ratnis possess a physical form. He has only been heard or seen when possessing another.
Strong willed, can possess the undead or the weak.
Has no body, requires a vessel to truly act.
Timeline of Events
Sometime during the Before Hylia era, Ratnis split away from his Ma brethren for unknown reasons that put him at odds with them. He vowed to one day usurp Demise and dominate the land in his place.
Ratnis has no known historical activity prior to AD-1200.
In AD-1200, after the events of The Storms Assault, Davus Fulmen was killed by Polaris. Prior to this, Davus had been sent back in time from the Interloper Era, and had been observed periodically by Ratnis. How Ratnis managed to see beyond his own dimension to watch Davus is not currently known.
When Davus was killed, Ratnis took the echoes of him and used his own strength to create a brand new Davus. This Davus was, at least from Ratnis' point of view, identical to the one who died in AD-1200. This meant that Davus' death led directly to his birth, ensuring that his circular timeline would continue to occur.
In AD-1560, Rhunerys Telaris observed a massive black thunderstorm bring lightning down upon the earth so grand that it left behind a man in its wake, and that man was the newly created Davus Fulmen. Born alongside Davus was Sirius Fulmen. Although the storm bore them, both arrived as fully grown men. Rhunerys used some kind of magic to cause Sirius to adopt actual human qualities and revert to a proper newborn form. Renaming him Fulmaren, Rhunerys then escaped, leaving Davus alone as the only tool Ratnis had to gather power in Hyrule.
Since this event, there has been no sign of any others created by Ratnis. Davus Fulmen roamed Hyrule for centuries wreaking havoc, until during the events of Interloper War when he was yanked back in time due to a deep ancient time magic linking him with Polaris.
There has been no further information made known about Ratnis since Davus was launched back in time. He is presumably refocusing upon Sirius, whose death and transformation into Morton has provided Ratnis a new window of opportunity.