Many people in Eiyuden Chronicle: Hundred Heroes are looking to discover the power of runes, but Code L is one you'll want on your side. He even helps you recruit another powerful character later.
You run into Code L in the Rune Shop of Daphan on the left side of the store. He tells you he's looking for the Rune of Currents. If you've already fought the boss in the Reveal Harlan storyline, you'll have it. If not, you'll need to make sure you have Carrie recruited and head to Euchrisse until you run into the monster. He will automatically drop the rune and add it to your inventory. Once you've done that you can return to Code L and recruit him!
After Code L realizes you're Nowa from the Alliance, he declares his disdain for how the dux is using the runes and decides to join you by opening a Rune Shop in your town. It can be found on the second floor of the Trading House to the east of the castle entrance.