His name is definitely "Maruo". The katakana is there because that is all we have, from one instance when Isanari called him by his name. Nakano's response was "Don't call me by my [given] name."
It is possible that "Maruo" has kanji, with the katakana used by Isanari to imply familiarity or slight slur or something. This is similar to the girls saying Fuutarou; none of them use the actual kanji 風太郎, it is written in katakana.
Don't think it matters if "Maruo" is an unpopular name. Japan is one of the countries with the highest number of unique names.