Rhoy the Explorer is an invader in the Grave of Saints if the Player is not a member of the Rat King Covenant and is offline. From the mentions of him in item descriptions, he was a prodigious adventurer with great presence of mind and a knack for evading traps and danger. It is likely that Rhoy was a servant of the Rat King, helping him defend his realm. He may have begun communications with the Rat King using his Ring of Whispers in a time before the King could speak the language of humans.
Rhoy appears to have travelled some extent of Drangleic and been renowned for it. There is a bonfire in the Shrine of Amana called “Rhoy’s Resting Place” and it is interesting to speculate on how it came to be known as that. Items associated with him that focus on preservation indicate that he was an adventurer who relied on what he already possessed for all situations.
The invader who attacks us in Grave of Saints wears the transparent Aurous set. An armor given to the kings of Jugo. He also wields a shotel and a great crossbow, and we know that he is wearing both the Ring of the Exalted and the Ring of the Conqueror because his weapons are invisible. These rings are all gained by overcoming overwhelming odds, and indicate that Rhoy was a very capable individual.
The Rat King mentions that the Crest of the Rat (the covenant ring) was created by a human. He also explains that the lockstones were not created by rats, but rather by a human hand long ago. It may be that he is referring to Rhoy in the first instance as the creator of the Crest of the Rat, and Pharros in the second as the alleged creator of the lockstones. Though, it should be noted that inventions attributed to Pharros are sometimes wrongly so.