Short answers, pulled from the story.
Guts was created by Kentaro Miura as the protagonist of the manga series Berserk. Miura first sketched a dark warrior with a gigantic sword while working briefly as an assistant to manga artist George Morikawa at age 18.
Guts' personality and appearance were partially inspired by Rutger Hauer's performances in Flesh and Blood, Blade Runner, The Hitcher, and The Blood of Heroes, as well as by Miura's high school friend Kouji Mori. His prosthetic arm drew from Hyakkimaru of Dororo and the protagonist of Cobra, while his oversized sword combined imagery from a character by Shinji Wada and an illustration from The Snow Queen, a Guin Saga spin-off.
During the Eclipse, Griffith sacrificed the Band of the Hawk to be reborn as Femto, a member of the God Hand, and the Apostles slaughtered the group. Guts lost his left forearm and right eye trying to save Casca, who lost her sanity from the ordeal. The Skull Knight rescued both of them and warned Guts that the God Hand had branded him.
Nobutoshi Canna voiced Guts in Japanese for the 1997 anime, with Hiroaki Iwanaga taking over for the films, the 2016 anime, and Berserk and the Band of the Hawk. In English, Marc Diraison voiced him in the 1997 anime and films, Kaiji Tang in the 2016 anime, and Michael Bell in Sword of the Berserk: Guts' Rage.
A study titled "Campeones Solitarios en la Viñeta Japonesa" compared Guts directly with Thorfinn from Vinland Saga and Miyamoto Musashi from Vagabond. All three protagonists had childhoods defined by violence and, as their stories progress, seek to move beyond that violence toward more mature and peaceful lives.
IGN wrote that Guts served as a template for many heroes who followed and that his oversized sword started a trend visible in characters like Cloud Strife from Final Fantasy VII and Ichigo Kurosaki from Bleach. Yoko Taro stated that Caim, the protagonist of the original Drakengard, was directly inspired by Guts, and the game Dragon's Dogma includes armor modeled on both Guts and Griffith.