Zelgadis Greywords (Zerugadisu Gureiwāzu) is a sorcerer who begins the various incarnations of the franchise as a retainer for his ancestor Rezo, opposed to Lina, but ends up in Lina and Gourry's group. He is a chimera, made of rock golem, brau demon and human.
Zelgadis tends to become something of an enigma to his comrades, due to his quiet and asocial nature. When he was around the age of a teenager, he was transformed by Rezo, the red priest—who is both his grandfather and great-grandfather (actually, Rezo is only Zelgadis's great-grandfather, the previous presumption made by a translation error to English)—into a three-part chimera.
Zelgadis specializes in Shamanistic magic, which pulls its power from the astral, or spiritual, plane. His elemental spells are formidable, and include Stone Spiker, a modified version of Ray Wing, and his trump card, the mighty Ra-Tilt. However, because he is part demon, he is unable to cast Black Magic, as doing so requires him to call upon the power of another demonic being which, in the Slayers universe, is akin to willing oneself out of existence. Though he is not as learned in the art as Lina, Zelgadis's even combination of physical brunt and magical finesse render him among the most fearsome warriors.
Zelgadis travels with Lina & co. because he wants to find a spell that can change him back into a human. Despite the incredible abilities bestowed upon him by Rezo in the transformation, he dislikes his change in appearance (his skin is stone hard, tinged blue-grey, with pebbles scattered across its surface and his hair being as stiff as steel wire) and wishes more than anything to regain human form. However, Zelgadis likes to consider himself evil, but not too evil. In one episode Zelgadis (and the others) are described in insulting terms. Zelgadis is called "the heartless mystical swordsman", an epithet he actually likes (Lina and Gourry are called "Some smartass sorceress and an idiot swordsman" while Amelia is called "a crazy girl").
During Slayers Evolution-R, Rezo informed Zelgadis that he knew of no cure to return him to human. It is unclear whether this means that Zelgadis will forever remain a chimera, or if Rezo's knowledge was simply limited in that respect. Hajime Kanzaka has however stated in past interviews that Zelgadis would probably eventually find his cure.
Out of everyone in the group, Zelgadis seems to gather feelings for Amelia. This becomes more clear in NEXT, as he gets extremely upset whenever she gets hurt, even going so far as to put his body between her and Gaav's sword and saving her, but nearly killing himself. In later seasons, they become quite close and he stops being closed up around her. Amelia returns the feelings and when they part before the end credits of TRY ep26 apparently give him "The Lady's Favour" in the form of one of the pink bracelets (with a blue gem) that form part of her standard slayers costume.