Assassin's Creed Valhalla

Nar unlocks in Odin's Camp after your first run and after buying some skills from the skill tree (I personally bought around eight or so after my first run, and then the shop for Nar unlocked on my way through camp to start a new run).

This achievement is cumulative across all runs. In other words, you don't need to buy all Favors at once. Instead, you need to buy each one at least once at some point during your time playing. I recommend buying the majority of them for your run when you attempt "Sacrificial Victory," so check that achievement solution out and you'll make the majority of progress there. Then, after that run is complete, you should have enough Memories and Thoughts to buy whatever you have left to purchase.

There are eight Favors to purchase for this achievement, six of which require Memories and two of which require Thoughts. Memories will come from completing encounters, side quests, killing bosses, and as rewards when specifically marked on your map. You'll get plenty of Memories just by playing. Thoughts are only awarded for beating the bosses, so they take a bit longer to amass, but after a couple runs, the first two bosses shouldn't be too difficult to routinely beat to get their Thoughts, so you can farm them there if absolutely need be, but you'll get enough Thoughts from playing naturally, as long as you don't waste them on Outfit upgrades that you'll never use.

Unfortunately, there is no way to track which Favors you've purchased and which you haven't, so make a mental note of which ones you have yet to purchase. Or, just purchase them all at once so you don't have to worry about it.