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"Do you really wish to know?"
Spoilers from the books and/or games to follow!

Giselher (d. September 1267) was the ringleader of the Rats. In the past, while he was drunk, he mistakenly enrolled in the army of the King of Geso. A week after this error, he deserted the army and joined a gang composed by others deserters. The gang started ransacking and burning to the ground villages and ambushing the Nilfgaardian Army until he was exterminated by a squad of Elves; only Giselher survived. He remained stuck to a tree with a arrow through his shoulder until Iskra, a elf woman mysteriously abandoned by her mates saved him and healed his wound; they later became traveling companions and later lovers. They then met Asse, Kayleigh, Reef and Mistle during the holiday of Lammas in a village of Geso and formed a gang. The entire gang then, with the exception of Ciri, was killed at the hands of Leo Bonhart in Jealousy.