The Stelets are the secret enforcers of the Gaius's will; removing threats to him or the nation from the shadows.
While the modern Stelets are a relatively young group - only being formed under the current Gaius Ilya, the custom of their existence dates back to the oldest of Ussuran customs. Originally, they served the function of ensuring all Ussura properly followed the feast days of Matushka and followed her will. Over the years, as authority shifted from the goddess to the Gaius, they came to serve the will of the leader of Ussura.
Normally, their ranks are few - numbering less than a hundred in all of Ussura - but since Gaius Ilya came to power, their ranks have grown more than a hundredfold. Part of this is due to Gaius Ilya being the first Gaius in centuries to stand up to the nobility and part of it has to do with the Montaigne invasion. Either way, the secret army of Ussura has become a large organization enforcing Gaius' rule. They've already ferreted out a secret plot by the nobles to remove Ilya from the throne (which they dealt with swiftly and permanently) and have made major blows against the Montaigne that crippled them for months.
Lately, they have been dealing with dissidents in the nation.
Stelets are survivors first and foremost. They know how to survive in the Ussuran wilderness indefinitely and kill from the shadows. They are expert snipers with their bows and almost instinctively know where to deal the most damage to their prey - animal, man, or army.