Originally Posted by
vortexrain2013
Go to original post
I had the same problem as you, but I may have figured it out. I am a completionist, and wanted to get everything in the game; this meant getting every piece of clothing and weaponry in the game, and upgrading them. Before I did this, I was missing a few of the momentos. After I did this, I checked to see how many momentos I had, and I had all of them. This lead me to assume that getting all of the weapons and all of the clothes, and upgrading them are how you unlock all of the momentos.