Rabi-Ribi: Is the order a DLC?

Summary pulled directly from Steam - Some text may only be applicable to Steam platform

What are Ribbon's friends up to and what's the source of that myserious aura under the island?

This adventure will bring Erina to previously unexplored areas way high up above Rabi-Rabi Ravine and deep into the mysterious depths of the Hall of Memories.

Encounter new boss fights with new bullet patterns, abilities, all new music and a special CG!

Obtain four very special items, some of which will involve other characters when used!

Oh, and with a functional wardrobe, Erina can now change outfits at home!

Explore the new chapters "Fairies Attack!" and "Finding Memories" in this DLC!