Aife is a young Picatavian huntress who can join your party shortly after your initial conversation with Caustantin mac Fergusa in the second act. She wants to join your hird, so she can observe your fighting style and aid you during Mac Taidg Must Die.
In case you side with Northumbria and you let her join the party, she will leave the hird at some point during a visit to the first campsite after Fort of the Thicket. During Into the Dark, you will find her at the side of Grianne, who you have to fight. In case you incapacitate her during the battle, she will be heavily wounded. If the party leader has some skill in healing, you can prevent her from dying from her wounds after the battle. Then you can take her as prisoner of war or let her go.