Gigant was clearly not a muscle-brain and showed great wisdom. He offered Cain a path to the capital and requested some help. While immensely powerful, he was one man and could not be in two places simultaneously, so he needed powerful and talented people to help him.
Cain did not have a problem helping, especially if, in doing so, he could carry on a personal mission.
"I am not familiar with your troops or terrain, so I would not be a good fit for rescue efforts or trying to dismantle territories, but there is something I excel at, and I assume you will soon be in need."
"What would that be?"
"Espionage and assassination."
Gigant's eyes narrowed as he heard Cain's specializations. Spies and assassins were essential to an empire's military power. Still, now their enemies were the environment and Wave Beasts, not targets that required the skills of a spy.
As if reading the Duke's mind, Cain smiled before continuing with his words.