Tolya Yul-Baatar, sibling of Tamar, was first seen in Siege and Storm. They were Heartrenders, and part of Nikolai Lantsov's crew. However, as the twins grow closer and more trusted with Alina, they become part of her personal guard.
Although they seemed unwavering in their allegiance, they were actually working with the Apparat. However, they had followed the Apparat only because they believed his claims of Alina being a saint. And so, as they had shown in Ruin and Rising, if they had to chose between the Apparat's ruling and Alina's, they would chose Alina.
Tolya, as said by many in the books, had skill with weapons only matched by his sister. As Botkin, trainer at the Little Palace said, he and his sister would never marry; their purpose was only to fight. Praised for his skill, size and talent, he was one of the best fighters there, and proved helpful in the war.