Merlin
Merlin (Japanese: マーリン Maarin) is a common Evolution medal in Kingdom Hearts Unchained χ / Union χ.
Two Merlin medals are used to evolve a 2★ medal to 3★.
You can obtain Merlin medals by completing the Low level mission in the Evolution Daily Dungeon on Fridays or by beating Guard Armor or Guard Armor Subspecies Raid Bosses.
Merlin medals can also be obtained by defeating Pretenders in quests 108 and 158 (more chances).