This game's tutorial is not that great, and I've kinda just been throwing units at people and seeing if it sticks. Still have no idea what the medallions do.
I agree with this assessment, the game is relatively simple, but after going through the tutorial, I ended up more confused than aware of what to do.
Medallions are used as recruitment tools and title fodder. The yellow ones in the corner can be turned into more medallions by taking up resources (the hammer things on the main screen). From my experience, resources are quite plentiful so don't be afraid to burn a few for titles, the spear medallion gives dragonkiller title in the first position which gives off slay dragon, helmet split:5 and a huge 16 bonus to atk early on. It's quite worth it to even fire some of your current units and rehire them just for this. Skill/Sprite medallions in the first position can give out Treasure hunt to various levels, Demon/Magic/Fauxmon in the second position medallions can give out Bounty Hunter (Ores) to various levels.
The very first medallion of any new type that you get (i.e rank 2 onwards) also unlocks new hires although it doesn't guarantee that you can hire that unit with your current set of medallions, making a dragon medallion unlocks the rightmost lancer(Silver Dragon) under dragons for example, but that dragon also requires a rank 4 medallion to recruit. As added information, unlocking higher rank medallions relies on a mixture of Conquest and Reign points, Reign will allow you to unlock 2 medallions faster, but the rest requires a total that is very high.
In line with what I said earlier about remaking units, here's a few of the medallions you need for units you may want to remake:-
Worm Queen - Poison
Lupold Lancer - Knight
Kobold Engineer - Sprite
Dullahan - Hero (Note that you need a rank 4 medallion to remake her)
Hannya Samurai - Hero
Phantom - Blood
I'd also throw out a quick personal plug for Dracaenaria who has a very useful skill, Rainbow venom which inflicts a whole bunch of status effects onto anyone it hits and doesn't require rank 4 medallions. Make a Blood medallion to unlock her.
Unrelated to anything above, let me also add some information I wish I knew beforehand:-
Exp/turn is given to all units irrespective of whether they've in a squad or not, that said, exp from actually fighting significantly outweights the exp/turn you get, A single fight can easily earn you 20-30 turns of idling. You can take advantage of this to keep cost down by utilizing less squads (although admittedly, this is only helpful for the first few chapters....). On a similar note, units kept in a squad that don't fight still enjoy your healing/turn as well.
The status of your units and items is imported in full into new game plus, so there's no real reason to spend time raising loyalty on lower rank units or trying to level up lower tier units. (although there's level caps linked to growth type to consider but by the time you're hitting the cap, you're done with NG and can do practically whatever you want anyway). For this reason, it might be a smart move to keep some high rank ores around so you can immediately sell them and get some gold income on chapter 1.
If you're trying to corrupt goddesses, the "love" event supersedes the corruption event, so be wary of how much mingling you're doing if you want to keep all options open. You should also spend the tail end of chapter 7 doing all the corruption/mingling you want as the final chapter(at least on the law/reign route) seemed to block me out of most of the events.
