Polaris has always known Sven's feelings towards him, though it is uncertain if he ever returned them or not. He cares deeply for his brother and watches over him above even while tending to the handful that his assigned deity that is Kimos (Deity of Mischief).
Sven holds deep respect for his older brother and also a romantic interest. It was when his brother left him behind and ascended to the role of a Guardian by the name of Polaris that Sven decided to go on his own misadventures in an effort to be reunited with his brother and ascend to be a fellow Guardian.
Polaris views Kohaku as his brother's defender and pact partner. As his former self, Sever, he had a pact partner himself, but in order to ascend he had no other choice but to relinquish their pact. Knowing this, he hopes that Sven will never ascend or ever find him again out of fear that he might have to endure the same pain of losing such a precious friend.
Kohaku knows Polaris as Sven's brother. He knew Polaris before he ascended and that the man is very upright and the type that can be trusted even if he is mischievous at times. Sever is Sven's brother after all. He knows that the man would never do either Sven or himself any harm and thus is quite calm towards him despite his typical rage towards anyone other than Sven.