Arbiter, also known as Sibling Dearest, is the 'Balancer' in the Dark Triumvirate. They are Non-Binary/agender, and should be written as such. (The major difference, however, is that Arbiter has likely always been Non-Binary) Essentially an empty shell, they act as an overseer who sees all.
Unlike the other members of the Dark Triumvirate, Sibling shows no outward emotion. They emote once in four million years ago, and the latest one is confirmed to be fear. However, they are certainly not softspoken and do not enjoy the company of Brother Dearest. They're smart, having millions of years stored in their memory. But there are limits to their ability to understand. Like Archivist, they are unable to comprehend human terms in much the same way a monkey is unable to make use of the laws of physics, even if they were imprinted on its brain. However, they are able to continue towards fulfilling their ambitions despite this.
Their role in the story
Sibling's Flesh is the first to be restored, by Sam and Daniel.