6. Marc Created Steven
In the comics, Marc Spector is very much the main character in the Moon Knight property. However, when the Disney+ series started, it was Steven Grant that was the focus.
Naturally, with Marc being on the outskirts and relegated to speaking through reflections, it was assumed that Steven was the original. This was never confirmed however, and when they were in the asylum and on the journey with Taweret to the Field of Reeds, they were forced to relive their worst memories in order to try and balance their scales.
This led to the reveal of the devastating truth. As Steven went deeper into his childhood, he found that it was not he that created Marc at all, but rather the other way around. Marc was the original host of the body, who created Steven (and who knows how many other personalities), as a coping mechanism for a truly horrendous childhood.
For all the humour and silliness Steven brought to the table, this realisation was one of the most heartbreaking and distressing moments in the entire series. His world had already been turned upside down, this could have crushed it completely.