He is a scholar and was very curious about the rest of the world. He had learned how to crudely disguise himself among humans by wearing a loose robe (because "humans have a habit of shooting deathclaws on sight"), and claiming that he was really a human but had some deformity he wished to hide, and traveled in the wasteland among them learning as much as he could about their cultures.
In 2241, he's studying the culture his pack is forming and comparing it to human development along similar lines. When the Chosen One met Goris for the first time, he said that he had only been at Vault 13 for a few months and has acquired all the information he needs there, and wants to go again for his research. He doesn't like to travel alone, so he offers the Chosen One to become his companion to help him in his research and wants to show that deathclaws can work with humans as friends, not enemies.
When the Enclave killed everyone in Vault 13, no matter if accompanying the player or not, he was thus spared of the massacre (because he was in one of his scholarly trips if not accompanying the player). After that, he can accompany the Chosen One to further study the world around him and to avenge his kin.
One of his particular characteristics is that he's affected by albinism, due to a different mutation with FEV. His hide is completely dark grey instead of earth-tone colors like other deathclaws, and his eyes are red. He also has a sensitive hide due to his albinism, because even when he is among his pack, he always wears his robe and only removes it during combat. He also possesses a telepathic sense connecting him with his brothers, seen when the Enclave attack Vault 13 if Goris accompanies the player: he feels his brothers are in danger and leaves the group to return to Vault 13.
Goris may join the Chosen One's party in search of new experiences and knowledge. His absence from Vault 13 meant that he was the only survivor of Frank Horrigan's massacre of the "rebellious" deathclaws. He, possibly along with Xarn (depending on the player's actions), are the only two surviving intelligent deathclaws but they can produce intelligent deathclaws, since the gene was made male specific and any intelligent male that mated with a non-intelligent female deathclaw would (most likely) produce intelligent deathclaws.
Normally, Goris wears his brown robe everywhere he goes. However, when combat begins, he dramatically tosses them off, revealing his deathclaw form. Goris is tough, quick and skilled in unarmed combat. However, his inability to use other weapons than his claws reduces his utility as an NPC somewhat (his tough hide is equivalent of combat armor).
Goris can communicate with low-intelligence player characters fully, which is fitting, since he lived with deathclaws of various IQ levels. Oddly, Goris doesn't mention that he is a deathclaw when he is recruited. This means the player only finds out when they enter combat for the first time.
Thanks to the combination of his extremely high strength and lack of weapons or armor, Goris can be used to carry heavy equipment much easier than most other followers.
It is unknown how to trigger this glitch, but sometimes after exiting combat, he does not put on his robe. However, when going into combat he puts on his robe but it has disappeared. It returns to normal after a while. [verified]