Aranea was a mercenary who was hired by Niflheim due to her unique fighting abilities. She was given rank and soldiers in order to see her missions through, including her two subordinates Biggs and Wedge. She attacks the party when they infiltrate a base, but leaves since she does not get paid after a certain time.
Later in an instance in which the party is cooperating with Ardyn, the imperials and Gladiolus has left to handle business of his own, she joins the party as they raid a temple in order to "train them", while really the goal is to obtain the mythril ore within for Cindy's friend in Lestallum. Together, they fight through legions of skeletons and creme brules and defeat Quetzalcoatl, the temple guardian, and retrieve the ore it protected. After the fight against the lightning bird, she suggests Noct watch his "princely/kingly" self, and that she will train the party some more if their paths cross again. Talking to her again will result in them being given a ride back.
Through her, the party learns that the empire is harvesting "specimens" to make into Daemons for warfare. Examples of this include the Magitek troopers, which are made from the parts of Iron Giants in specialized laboratories. She disagrees with this practice and is seriously considering leaving for good and hunting the daemons down as a mercenary again. She also does not approve of the Emperor and his new high commanders, and especially detests Ardyn.
When the party arrives in Tenebrae, she lends her aid once more by arranging for Biggs and Wedge to take the party to Gralea to recover Prompto and the Crystal.