The Royal Rat Vanguard looks very similar to his Corpse/Hunting Rat brethren, except his eyes are yellow, his mohawk fur is more pronounced and his ears are bigger. He is a loyal servant of the Rat King and judges the worthiness of those whose who seek royal audience by facing them in combat. If defeated, the winner obtains a Rat Tail and therefore the right to serve under the Rat King.
The Royal Rat Vanguard’s soul description states that it is a “living statue”. The Corpse Rats (Hunting Rats) serving under the Rat King inflict petrification, as do the Dog Rats in the Doors of Pharros. According to an old local legend, a great spear was used to defeat Santier, the walking statue. Perhaps it was that Santier too was one of the Rat King’s subjects, defeated by one who would seek the right to serve. The Rat King himself confirms that there was “a human who was once among us”, so perhaps we are not the first to kill a living statue and thereby earn the right to serve this eminent being.
It may be that the Rat King has the ability bring statues to life in his attempts at ruling the lands not lapped by the Sun’s rays. We do see a number of rat statues in the Grave of Saints and the Royal Rat Vanguard’s boss chamber.