Sigurd was originally a somewhat fragile person, who only had his fiancee, Aina, to keep him mentally in one piece.
Found by Rou in a village holding the upper half of his lover, he was overcome with rage and fought with Rou. After being defeated by him, he was offered to be his subordinate in exchange for him helping in obtaining the power to avenge his lover.
He was one of the main characters in this world, chosen one by the gods like Rou, who had his own storyline written in the Heroic Psalms.
He is now part of Rou's, whom took over as his "god" and now manages his abilities and controls his growth.
Sigurd was rather an skilled swordsman due to his hero training on the city but his personality made him hard to deal deadly blows. After succumbing to his vengeful thoughts and turning into the Avenger, his senses seem to be sharper and has no doubts on cutting through.
Furthermore, he is a Hero with his own Psalm, which means that has access to really powerful abilities though, now that his Psalm is controlled by Rou, it's not longer up to him whether or not to trigger those skills.
Aina was his fiancee.
Initially, his relationship with his group was rather dull since he perceived them as nothing but a tool to increase the rate of success of his revenge. Now that he has fulfilled it, it's unknown if his point of view about them has changed.