Delphinus is an optional boss found the the S Rank section of the coliseum in Hyperdimension Neptunia mk2. She is described as the game's hardest boss, but not the final.
Delphinus is a pallette swap of DOS Arfoire. She has light pink wings, skin more human-like, blonde hair, a golden double-sided spear, and silver dress.
Delphinus is referenced a few times in IF's dialog throughout the game, and has a namesake in IF's 'LaDelphinus' SP move. However, the same SP move also denounces Delphinus as a being that IF made up, so the Delphinus fought in the coliseum may be the manifestation of IF's Imagination - this fits with other presumably phantom enemies that can be battled in the coliseum (like the shadow and fake party members), and if it were the the manifestation of IF's imagination, it would not be unreasonable to assume that it would take the form of the most powerful being IF had encountered - namely DOS Arfoire.