Argilla is one of the five members of the Embryon who feature predominantly in the Digital Devil Saga games, and the only female among them. More specifically:
Digital Devil Saga
In Digital Devil Saga, Argilla is one of the first people shown to be gaining her emotions. When it is discovered that the strange things that are happening to them are because of them turning into demons, she takes a moral standpoint and refuses to eat other demons, even after being assured that it is a new "law" within the Junkyard. She eventually relents on this stance.
Argilla seems to feel a kinship with Jinana, who shares Argilla's feelings of disdain towards the new way of the Junkyard. When Jinana is betrayed by a member of her own Tribe and starts losing her mind to her demon form, Argilla initially hesitates to fight her.
Her refusal to eat those she has killed leads Argilla to be weak for a considerable while, and prevents her from learning skills for a while.
Throughout the rest of the game, Argilla shows that she has come to embrace morality: she also will not listen to plans of betrayal. She seems at odds with Heat on occasion, due to his brashness and apparent temper.
In combat, Argilla is initially forced into your party alongside Serph and Heat, and is strong with tera spells and weak to zan spells. She has a high magic stat, making her suited for magical offence or healing.
Digital Devil Saga 2
In Digital Devil Saga 2, Argilla is once again forced into the initial party, this time with Serph and Gale. She is sincere about wanting to help people, and seems to feel sickened about the state of "Nirvana".
Argilla decides to risk her life to help Roland in fighting Metatron so that the others can escape from the facility they are trapped in, and is seen getting into a precarious position due to this.
It is also revealed that the "original" version of Argilla (before she became a denizen of the Junkyard) worked alongside Heat and Serph as a scientist. She was manipulated by Serph to kill Heat when he tried to interfere in their treatment of Sera, showing that her emotional characteristics have perhaps come from this other life.