Sif is the wolf companion of Sir Artorias, the Abysswalker. After Artorias passed away, Sif became the guardian of Artorias' grave in the Darkroot Garden, to prevent others from meeting the same fate as his former master.
A younger Sif appears in Oolacile in the past. He accompanied Artorias on his quest to save Oolacile from the forces of the Abyss. However, Artorias and Sif were overrun, and the former used his shield to erect a barrier around Sif to protect him before he became corrupted.