The setting is medieval, a magical fantasy world, you go on an adventure as a novice adventurer, you are quite immature but you are very lucky and find an ancient library, a brief about it. Yui, a Tome Keeper of the ancient library you found, her age is 26 but has stopped growing and she is immortal, the ancient library dates back 600 years, so since then, Yui is probably 626 years old.