”It took hours to decide, but Restares is right--this is what must be done. For the good of Alethkar.—Amaram to Kaladin, on deciding to take the Shards for himself
Restares is a member of Sons of Honor, possibly Meridas Amaram's superior, and likely one of the organization's leaders. Restares may have originally brought Gavilar Kholin into the organization.
Gavilar briefly suspected on the night of his assassination that Restares tried to kill him, but never thought that'd actually happen.
Restares had a spanreed to Amaram, which was sometimes carried by Amaram's carriage driver, Pama. After Kaladin saved Amaram's life and kills Helaran Davar, Restares advised that Amaram take the Shardblade and Shardplate, and to kill the witnesses and pretend that Amaram personally won the Shards. The deliberation took hours, but eventually Amaram agreed, thinking it had to be done for the good of Alethkar. This resulted in Kaladin's enslavement, and the deaths of the last surviving members of his squad, Hab, Reesh, Alabet, and Coreb.
Mraize knew about Restares and referred to the Sons of Honor as Restares's cronies. Upon receiving a page of text from Shallan Davar about what Amaram knows, Mraize believed the Sons of Honor are also close to the secrets of the Shattered Plains and Urithiru.
After the Battle of Narak, Amaram wrote to Restares via spanreed that the Sons of Honor are successful in their goal of bringing the Voidbringers back to Roshar, with the hopes the Heralds would also return. Amaram informed Restares of the Everstorm and the Voidbringers' red eyes.
The following link contains major spoilers for Rhythm of War. Please be very sure you have either read the book or are happy to be spoiled on some major points of the story.