Caelus appears in Hyperdimension Neptunia mk2 as a CPU from many years ago who slumbers in LAN Castle's Secret Hall. Not much is known about her except for the fact that she and the other CPUs in her time attempted to defeat Arfoire and ultimately failed.
Story
Hyperdimension Neptunia mk2 and Re;Birth2: SISTERS GENERATION
When Nepgear and the others arrive and head to LAN Castle Secret Hall to find Caelus, she is awaken by their loud voices and asks them why they interrupted her slumber. When Nepgear and the others explain that they need her help in trying to defeat Arfoire due to her being the only remaining CPU from the past who previously attempted but failed, Caelus decides to comply and offer advice.
Caelus states that if they truly wish to defeat Arfoire then they must all work together, looking into their hearts and combining the power of their wills towards the same goal; defeating Arfoire and restoring peace to Gamindustri. She adds to this by stating that she and the other former CPUs did not put their differences aside and act in this manner which lead to them being unsuccessful. After saying all of this, Caelus resumes her slumber.