Medals

"Taken from Mistive on Reddit"

What are Medals?
When you sell a 3 star, 4 star, or 5 star unit in game, you'll receive two types of currency: Gimmel, the gold used in game to upgrade, enhance and evolve your units, and Medals, which can be used to buy special items in the Medal Exchange found in the menu. Currently, this is the only way to obtain Medals in game right now. You cannot exchange Grimoires for Medals. It is not yet known if you can trade items bought in the Exchange in the upcoming Bazaar.

Units and Medal Conversion
The amount of Medals you receive depends on three factors: the rarity of the unit, the evolution of the unit, and whether it's loyalty is maxed. As of this guide, I personally haven't sold any 5* or 6* units, so I can't say for certain how much they'll give.

Medal Exchange
The exchange features many exclusive items and other niceties such as battle items, Grimoires, units and even unit storage increases.

This list can also be found in game.

Note: Barca is a 3☆ unit, Leraju is a 4☆ unit, and Ishtar is a 5☆ unit. Extra Cache refers to storage space on your unit list.