When it comes to battle in Eiyuden Chronicle: Hundred Heroes, Glen is more confident in his card skills than in his actual combat. However, the game is fun to play and provides entertainment for your characters and the rest of the town.
When you go back to Arenside after you have fast travel, return to the house you had your first conversation outside of on the west side of town. You'll see Glen and a young boy talking and interact with them. He will teach you how to play the card game he and the boy were playing and if you beat him he joins you at the castle.
In the Trading House to the west of the castle entrance, Glen opens a card shop on floor 1. At the Card Shop, you can battle, buy new cards, form your deck, and view your cards.