We first saw Skull knight warning Guts right after he left the Band of the Hawks about the group's approaching demise. His horse appears to be able to fly and jump over trees. In the course of Guts's journey, he saved him several times and aswell seemed to alter the course of events around Guts for his own purpose of revenge. He alleged Guts is a "struggler", who always is struggling to oppose fate.
The Skull Knight kills apostles and monsters easily and can hold his ground even with Zodd in his demonic form. He swallows the behelits which he recovers from dead apostles. In accomplishing so, he allegedly destroys and reforms them as his Sword of Resonance, which can cut through space and time dimension.
Major Story Arcs
Before entering the The Eclipse Ceremony skull knight first fough Zodd, whom is said to be his adversary for quite some time. After defeating Zodd, skull knight rushes into the ceremony, and interrupts As he was saving Caska and Guts from the God Hand, he quickly attempts to attack Void, with little success, as Void uses his alternative dimension powers, which he can do it at will to protect himself, sending the Skull Knight's sword traveling into the portal and advancing out again, back towards skull knight again. After Void and the Skull Knight barter glances for a moment, Skull Knight quickly goes to save Guts and Casca from harm.
The Skull Knight again dispatches abounding Apostles in seconds. Femto again attempts to stop him from escaping by using his power, which the Skull Knight dodges easily before heading straight out of the window he came in. Zodd yields allowing Skull Knight to take Guts and Caska, as a mark of respect for being defeated in battle. The Skull Knight is the mysterious character holding a lot of knowledge about the ways of the world. He keeps to himself and has only talked to Guts and Flora the Witch.
Skull Knight appears gruesome but is very caring. He occassionally aids bystanders ignored by Guts. Skull Knight regrets like Guts for not acknowledging what he had until it was gone. While Guts travels for revenge the Skull Knight desires atonement. Skull Knight has lived over thousands of years and holds vast knowledge. He kills apostles with ease except Zodd who seemed to be an unofficial rival. Skull Knight seems void of emotion mostly likely done by using the Berserker Armor before Guts and using the mysterious armor he now possesses.
During the Conviction story arc the Skull Knight appears briefly, mainly to warn Guts about the incarnation Ceremony and God Hand. He also tells Guts that he can slay Griffith or he can save Casca but he can not do both. Beside this he is also fighting the Egg-Shaped Apostle.
Millennium Falcon Arc
The Skull Knight enters Qliphoth to help Guts who is being slowly controlled by Slan. The Skull Knight distracts Slan long enough for Guts to attack her with is arm cannon then his sword the dragon slayer. Qlipoth then starts to collapse, the skull knight uses his sword of resonance to help them escape.
He is later seen talking to Flora about giving the Berserker armor to Guts. Flora tells him not give it to him but the Skull Knight says he has no choise because Griffith army comes to destroy them. He is later seen fighting Zodd again. He witness Guts use the armor against a transformed Grunbeld.
Later he is seen telling Guts that he can bring Casca back to normal, but it may be something Casca does not want.
The Skull Knight uses his sword to teleport behind Griffith to attack him but Griffith moves out of way and instead hits Ganishka. This causes a beam of light to shot across the world and the real world and the fictional world collide causing many fictional animals to appear like fairies, dragons and ogres.
Not much is known about Skull knight and his past. Princess Charlotte already told of a adventure about an certain Emperor Gaiseric who ruled his kingdom generously, given that it was a short lived kingdom, as it was perished beneath the Midlands by a God. Gaiseric was said to worn armour which bore resemblance to that worn by the Skull Knight himself, with the inclusion of a skull-like helmet and heavy armour, which makes Guts think of the possibility that Skull Knight and Gaiseric might indeed be the same person.
Also, it is hinted that he sacrificed the kingdom he built for his transformation into an Apostle. he battles against evil, and Slan addressed him as "King". He has known, and allegedly had a close relationship with the witch Flora for several hundred years and was also one of the previuos owner of the Berserker Armor.