D:OS keeps track of the relationship between the main characters (dual dialogues) and the opinions of NPCs, etc. Dialogue effects character development and quest progression, and vise versa, so characters can not be just swapped in an out, as they can in games where character development is completely separate from the state of the game world.
D:OS 2 will have a greater focus on inter party dynamics, but there are things that could be done to allow some customization when taking over another character part way through the game, etc.