Nar's Favors becomes available after you assign a skill using the totem near the starting point on Odin's Island then perform a run. After that he can be found in Odin's Camp, a Favour can be used in the next run.
Not all favours need to purchased in one go, they can be purchased for different runs.
Below are the favors, their description and the cost:
Elk Tracker - 50 Memories - Reveal the locations of all the Elk Shrines on the World Map
Silent Assist - 50 Memories - The sentry sounding the horn that triggers the Helfire in Dokkerland will stay silent
Vivid Memory - 75 Memories - Recover one of the Abilities you had at the end of the last run. Receive it along with your starting gear. The ability will be automatically mapped to A.
Keepsake - 75 Memories - Recover one of the weapons or shields you had at the end of the last run. Recieve it along with your Starting Gear.
Elk Heard - 150 Memories - All regions have an increased number of Elk Shrines.
Inflated Currency - 150 Memories - merchants offer a 30% discount for items that require Coins or Gems
Awesome Advantage - 15 Thoughts - Rune rewards you receive will only contain Legendary Runes
Fames Fortune - 15 Thoughts - Some weapon chests you open will only contain Legendary Weapons