Amaru is one of the Choquequirao guardians along with Mallku and Misi. Amaru is a serpent deity. He is the most level-headed of the three and is the reason Sadie and Felix do not have to undergo any more trials. His name is derived from the quechua spelling of 'Amaru'. Really, Amaru is just the name of his form of deity.
An Amaru is a mythological creature often depicted as having a snake-like body with wings. They are also sometimes shown with the head of a puma and a bird. Since Misi is a puma deity and Mallku is a bird deity, Amaru is a culmination of the two and a serpent. That is most likely why he seems to be the authoritative figure; he embodies the other two deities and may be more powerful because of it.