EX Skill Master Achievement in Octopath Traveler II

Each character has two EX skills to learn, which are awarded by 1) completing their story and 2) by interacting with their EX alter. Obviously the first one is self-explanatory, and you can use the video below to find all the EX alters. Make sure the proper character is in your party when visiting the altar to learn the skill, as only the primary job class holder will learn the EX skills.