If you really think about it in every game in the early 2000s everything really went through Shadow, Sonic Adventure 2 was all centred around him, he was the big bad up until the final moments and he was the hero who saved them all in the end, heroes was the only game which he didn’t do much but he still led the charge against Metal Sonic to allow Team Sonic to charge up into their superforms, not to mention he literally had a game for himself and he was the main threat to the main villain of Sonic 06. If you think about it apart from heroes Sonic was really just Shadow’s accessories in the final boss battles in the early 2000 Sonic stories.