What is the best use of reserve characters?

Personally, I use the character reserves I have to level up the skills of the characters that I use/like until they're maxed. Then I use the extras for adventurer medals. Usually, you want to follow that pattern with at least the 3 and 4 stars you get until their skills are maxed since they're harder to get. Doing that with the 2 stars isn't as necessary unless you're a beginner and don't have many 3 or 4 star characters. Then it makes the gameplay easier for you until you get higher rarity characters. This is all based on my personal experience and opinion but I hope it helps. 😃