According to Bingo Morihashi, Credo was originally planned to be the final boss of the game, similar to Vergil in Devil May Cry 3: Dante's Awakening. The idea was abandoned due to writing complications.

"We actually planned to make him the final boss at one point. Final bosses are rarely young men, so that would have been exciting. But that would have been really hard on poor Kyrie. and she would come to resent Nero, so we realized that Credo shouldn't be an out-and-out villain. Besides ruining Nero and Kyrie's relationship, making him a villain would have been a rehash of the "family betrayal" plot from "DMC 3". So rather, he starts off doing questionable things but later has a change of heart. Credo was easy to craft, in that sense."
Contributed by DrakeVagabond