Seranni is a Dalish elf and Velanna's younger sister. She first appears in the Silverite Mine in Dragon Age: Origins - Awakening.
Background
Involvement
During The Righteous Path the Warden-Commander is captured by The Architect and held in a cell. When the Warden wakes up, Seranni shows up and provides the key to the cell door. If Velanna is present, she will tell her that she is sick and they need to get with her out of here. However, Seranni dismisses her sister's warnings. If Velanna is not present, you can guess that she is Velanna's sister and may reveal to Seranni what became of her (however you cannot admit that you were the one who killed her).
Then, she notices that the darkspawn are coming to check on the prisoners and urges you to escape. The Warden can then persuade to tell her more of this place (Master Coercion skill is required) and she gives the key to the Architect's chests.
If Velanna is present at Drake's Fall during Depths of Depravity, Seranni will emerge from the shadows and a brief dialogue between her and her sister will occur, before she disappears into the dark once more.
If the Warden takes Velanna along and allies with the Architect against The Mother, Velanna will remain with the Wardens, but years later, on an expedition in the Deep Roads, Velanna takes off on her own after saying she had seen Seranni in the shadows.