In the first three volumes, he was depicted as a tall, muscular male with a huge sword called the Dragonslayer, and a prosthetic left mechanical arm that has a magnetic grip and also conceals a cannon & repeater crossbow. He is obsessed with his task, having no feelings or sympathy for any others who came across his path or get caught in the cross-fire. Guts did not flinch from using the Count's daughter as a hostage to kill him. He was always attacked by evil spirits at night, which gave him no rest due to the sacrificial Brand on his neck.
In the Golden Age Arc, more of Guts' past was revealed, explaining why he had become so obsessed with killing and revenge. He is born as one who may be able to struggle against causality. Griffith
He is the founder and leader of the mercenary army Band of the Hawk. Extraordinarily charismatic and handsome, his tactical skill gave him and his army the reputation of invincibility, and the favor of the King of Midland, who was locked in a century-old war with the Empire of Tudor. Due to his many important victories in this war, he is made Lord Protector of Midland by the king, despite his common ancestry.
Griffith was willing to sacrifice everything for the dream of his own kingdom, believing that he is destined for things greater than the average man. Early in his career, he won Guts' service in a duel and Guts became his sword in both the battlefield and in his political fight with disgruntled Midland nobles. A large part of his success was due to Guts' fighting ability and Griffith grew to be dependent on Guts, although their relationship was not exactly friendship.Caska
She was born and raised in a poor mountainous farming village along an unspecified border between two warring nations (possibly Midland and Tudor or Midland and the Kushan Empire). Her parents sold her into a local lords' service at a young age, but he attempted to sexually assault her before they could even reach his estate. She was rescued by Griffith and the first members of the Band of the Hawk; interestingly, Griffith did not rescue her directly, but lent her a sword which she used to kill the noble. With nowhere else to turn, she learned to wield a sword and joined the Hawks, eventually rising to the rank of commander. Until Guts arrived, she was Griffith's Second-in Command. Skull Knight
The Skull Knight first appeared to warn Guts after he left the Band of the Hawks about the group's future demise. His horse appears to be able to fly and jump upon trees. Over the course of Guts' journey, he saved him several times and also seemed to manipulate events around Guts for his own purpose of revenge. He called Guts "struggler", who forever is fighting against fate.
The Skull Knight kills apostles and monsters with ease and can fight evenly with Zodd even in his true demonic form. He swallows the behelits which he recovers from slain apostles. In doing so, he apparently destroys and reforms them as his Sword of Resonance, which can cut through space and time dimension. Zodd
Zodd, also known as Nosferatu Zodd and Zodd the Immortal, is a powerful Apostle. He resembles a 7-8 foot barbarian warrior in human form, and a humanoid Bull-Lion nearly twice that size when transformed. He also has wings which he can make appear and disappear at will, allowing him to fly.
He wandered from battlefield to battlefield seeking ever-stronger opponents and is known as a rival of the Skull Knight. He is considered by many mercenaries as the "god of the battlefield" and has lived hundreds of years without meeting a worthy human opponent.
There are, of course, more characters, more story and more everything. This is a long running series after all. But I don't want to give too much away in case anyone is interested.