You're talking about only having a single character, right?
From what I can tell, the game's story is based around two main characters, and the combat is balanced around having a party of four (with increasing difficulty as the party size shrinks). It would be quite a bit of work to rebalance every combat encounter in the game. Plus some puzzles are based around having two people.
nope Heart of Fury mode does not make you play only solo, it gives like 5 times more xp for entire party! In a video guy just made his LP solo for the challenge and to have as much xp for a one character, but xp would scale if he had like 6 in a party.