There are 15 medals to unlock in the game. Each one will also unlock a customization item, something that you can use on the character customization menu.

Aqua Man
Get 3000 points or more in the fishing mini-game - Awards 'Red Skin'.

Awakened
Complete the game - Awards 'Eyepatches'.

Diglet
Dig up 3 hidden items - Awards 'Black Pearl' and 'Thunder Earring'.

Divine Intervention
Meet Douglas Gavelin in every city - Awards 'Ghost Mask'.

Flawlessly
Fight 20 Flawless battles - Awards 'Devil Mask'.

Legend
Fight 50 battles - Awards 'Top Hat' and 'Demon Eyes'.

Lucky Charm
Get 1 Lucky hit in battle - Awards 'War Paint Tattoo'.

Lucky Charms
Get 10 Lucky hits in battle - Awards 'Gray Skin'.

Juggernaught
Complete the Summoner's challenge - Awards 'Eyebleed Tattoo' and 'Eyeshadow Tattoo'.

Rookie
Fight 10 battles - Awards 'Bandana' and 'Bloodshot Eyes'.

Santa's Little Helper
Complete 10 sidequests - Awards 'Anonymous Mask'.

Shrooms
Collect 50 mushrooms and/or butterflies - Awards 'Serpent Nose'.

Soul of a Poet
Find all 3 poems - Awards 'Masquerade Mask'.

Swordfish
Obtain all swords - Awards 'Masked Cape'.

Veteran
Fight 25 battles - Awards 'Fedora Hat' and 'Undead Eyes'.