Pluto uses that name as his favourite of the more recognisable Earth names for the ruler of hell. He does not like being called Iblis, Lucifer, Satan, Varuna, Angra, Hel etc. and is indifferent to "Hades".
Cerberus, who was partly born from the Greek legend (and shaped later forms of it), uses the Latinised spelling - as opposed to Kerberos or similar - to fit in with his master's name.
Pluto manifests in a fairly standard configuration for a demon. His body is regal, refined and wise in aspect, unostentatious but very well-designed. It is apparent that he is an old demon, but there's no infirmity; he is sharp and venerable right up until the end.
As with most traditional demons, he shows no obvious primary or secondary sexual characteristics. He takes the male pronoun purely as a courtesy to visitors who insist on labelling him one way or the other.
Cerberus is a relatively large, relatively lean and strong, short-haired hellhound with an obvious mutation: three heads. All three heads have working muscles, eyes, ears and the rest. However, Cerberus has only one mind.
Pluto and Cerberus have shown remarkable stability in surviving this long. (Time is a tricky area in hell, and for demons who spend most of their time at home, there is no way to translate hell's time to a useful Earth-year equivalent.)
Pluto is invariably calm and quiet. He receives visitors graciously and always seems pleased to talk. Cerberus is quiet and dignified, not ferocious as in his parent legends.
Although Pluto took elements of his persona from myth, there's no Proserpina or any other 'Mrs. Pluto' around.