Goris is a humanoid figure wearing a long-sleeved, hooded robe. 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 studied the culture his pack formed and compared 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 does not like to travel alone, so he offers to be a companion of the Chosen One to help him in his research and wants to show that deathclaws can work with humans as friends, not enemies.
Depending on the actions of the Chosen One, the Enclave will invade Vault 13 and kill everyone in it. No matter if accompanying the Chosen One or not, he was thus spared of the massacre (because he was on one of his scholarly trips if not accompanying the player character). 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, as seen when the Enclave attack Vault 13 if Goris accompanies the Chosen One: 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 Chosen'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. The animation of Goris taking his robes off takes around five seconds, as does the animation of him putting his robes back on. As a companion, 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, though 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 does not mention that he is a deathclaw when first recruited. This means the Chosen One 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 companions.
Sometimes after combat ends when he is either knocked down, out or unconscious, Goris might not put on his robe and end up walking around without it. However, returning to combat makes him put the robe on again.
The following is based on unverified behind the scenes information and has not been confirmed by canon sources.
Goris' motivations are caused by his fascinations of empires and civilizations that formed only to fall and was trying to discover the one common element among civilizations that eventually led to their downfall so he could devise a sort of 'cure'. But he was incapable of ever doing so, for he was trying (in vain) to understand humanity from the standpoint of a total non-human.
End of information based on unverified behind the scenes information.
Under some circumstances, after the deathclaw inhabitants of Vault 13 are killed, a bug will deny recruiting Goris. After recruiting him he will instantly try to return to the Vault, setting up an infinite loop of asking him to join you and him returning. This can be worked around by pressing 0 right after clicking on the conversation option to recruit him. This instantly cancels the script that returns him to the Vault.[verified]
Another way of fixing the glitch is to ask to recruit him then leave the vault and go somewhere else (NCR as it is the closest) and wait a day or two, then come back to the vault and recruit Goris and this time it should work.