This DLC offers extra content for Pampas & Selene that includes Skins, Pets, Special Games Modes, a Digital Game Manual, and a Special Developer OST. (Those who purchased and started the game in the first two weeks of it's release, aka Founders, already have this content as a bonus for free!)
This DLC includes:

Pets:

• Hada the Fairy
• Maggie the Cat
• Tod the Toad
• Cerdo the Pig

Skins:

• Jesse Fox [Pampas]
• Porkchop [Pampas]
• Zera [Selene]
• Ghost [Selene]
• Kleopatra [Selene]
• Cutezilla [Selene]

Game Modes:

• Explosions [Exploding Barrels!]
• Coins [No arrows or mana, only riches]
• Fast as wind [Run like the wind, unless you're hit!]
• Vampire [Drain life from kills and lose life over time]
• Hardcore Mode [You get one hit]

Special: 

• A digital version of the GAME MANUAL from the MSX computer version of the game.
• A Developer OST where Fran, the developer, sings the entire soundtrack a capella.