According to the game's writer, Bingo Morihashi, Nero was originally envisioned as a noble and sophisticated demon hunter, akin to the likes of Sherlock Holmes. However, the idea was rejected, and the team heavily debated the type of character Nero should be.

"I think he came out all right in the end, but he actually started out as a Sherlock Holmes-type character, sipping tea as he fought off demons. (laughs) But they decided that wasn't such a good idea. (laughs) That was the angle I was gunning for, though. He wound up being your typical wild and crazy kid, but in that sense he resembles Dante quite a bit, so I thought we needed ways to distinguish between the two. I gave him one part Dante's wildness and one part Vergit's sophistication, which gave him a refined, noble nuance. But that wasn't well-received either. So I was told to simply write the scenario as if the character was Dante, and once the team was satisfied with the story's direction, we began to flesh out Nero; his childishness, his unexpected serious moments. I believe this anecdote has been mentioned in some interview somewhere, but the team debated over whether Nero would be the type to cover up his privates in a public bath or let it all hang out. The base for Nero's design is undoubtedly Dante, and if you aged him ten years, Nero would basically resemble Dante physically. After a lot of discussion, though, I was convinced that Nero would definitely cover up his junk in a public bath, whereas Dante wouldn't have ever cared about that from a young age, and it wouldn't make a difference to him as an adult in "DMC 4". either. But Nero begins as a more reserved young man, so the big questions were, 'Is that enough of a difference?' and 'Are we implying that he'll grow up to be just like Dante?'"
Contributed by DrakeVagabond