Medals are unlocked at lvl19, together with Lair Hunt.

Each Medal provides with some specific amount of stats when used on a team, without needing to be activated in-battle. But each Medal has a specific buff/debuff when activated:

• Healing Medal - Removes all negative effects from your pokémon and restores a bit of HP
• Blaze Medal - Deals some damage to the enemy, and reduces their P.ATK by 50% for 3 rounds
• Protect Medal - Gives your Pokémon a bost of SP.DEF
• Torrent Medal - Deals some damage to the enemy, and reduces their SP.ATK by 50% for 3 rounds
• Shield Medal - Gives your Pokémon a bost of P.DEF

Each stat can be upgraded up to level 100. Upgrading requires Energy Crystals that can be obtained on the Lair Hunt.

There's also the option of Medal Refining, which upgrades your medal with more stats and also specific buffs for each medal:

This Medal Refining can also be upgraded up to level 100. Refining requires Medal Essence: Beginner Medal Essence, Intermediate Medal Essence and Advanced Medal Essence, which can be obtained on a specific event on the Island Tour.

This event is not always available tho, it's usually available every 3 months, unless there's a change in the Island Tour rotation.