The Equipment screen allows you to create decks for the four player slots you start with. By default, Agu is in the first slot. And there are additional slots for Kaputo, Lubecbea, and Tonedoro.
After you've finished creating a deck, you'll be able to try it in the Endurance or Simple modes.

Your deck has 12 slots for skills. A skill can be repeated in up to 3 slots.
You start with four skills: Fire Bolt, Water Bolt, Wind Bolt, and Ground Bolt. These are all simple attacks with easy-to-use commands.
Also, you can scroll through details of a skill. It's very helpful when you upgrade skills in Endurance mode, or when you review skills that other characters have.
By default, the card list shows you the names of skills. If you prefer to view the icons for their effects instead, toggle it with the Info feature
After you earn more skills, you can filter them by element or by command length.
You can equip up to 3 gifts. They bestow passive boosts.
You start with the shining stone. It gives you an extra chance in battle, so the opponent will need to win three rounds to win the match (instead of two).
You can select which player slot you want to modify in the top-right corner. By highlighting the character's image on the left, you can choose a new appearance.

Earn more skills by befriending characters in Endurance mode. For example, Tetori can teach you how to use Metal Bolt, Ice Bolt, and Release.
Earn more items by completing challenges. For example, you'll earn a wakaba patch for completing the tutorial. You can also earn items by improving your relationships with characters.
Also, by befriending characters, you'll unlock them in the Endurance and Simple modes. Playing as them is a great way to learn different styles of battle right away!