We have to ask ourselves why Zodd helped Guts when it comes to Berserk. He was there for Griffith last year when he was using new demonic power to unify the worlds of demon and man. We don’t know how this demon helped Guts. We do know that Zodd wasn’t an ordinary apsotel.
It was probably because Corkus was his ally and he was a mercenary. He was his adoptive father and taught him how to fight. He had a dark past and gave him his first scare when he became insane. He also tried to kill Sys by drinking one night, blaming Guts for his death. Guts was able to kill him with his sword. This traumatic event has remained in his mind throughout his life.
Guts clearly cares about Casca. He has found out from Skull Knight that Casca has a mental problem and needs help. Guts’ group is determined help Casca overcome her pain, and become a better person. They do this by entering Casca’s mind during a ritual performed by Hanafuku. Farnese promises Casca the strength she will need to deal with the pain in return.
Later, the Count appeared again through Vargas. The Count had been a savage and had sentenced countless innocents to death for heresy. Guts was able to get the behelit of the Count in exchange for his assistance. It was stolen from Vargas as Vargas fled the castle. The Apostle almost died when the Count had to fight Guts to get the behelit. The Count was unable stop Guts from activating his behelit and killing him after he had soaked the behelit in his blood.
Guts is able to control the Berserker Armor, which allows for him to keep his sanity while wearing the armor. However, there is a catch. Initially, Zodd disliked Guts and his party. Over time, Zodd develops a slight attraction to Guts. This is evident in his dislike for Bonebeard which is also reflected in scenes where he hides behind gunwales and chews on the banister.
As the relationship between Guts and Casca grows, Zodd is also involved. He is the first one to notice the bond between Guts and Casca. During this time, he helps Casca in her journeys to save the world. But when the two are faced with a difficult situation, he often becomes the calm voice of reason. This trait is displayed throughout the trilogy.