1. The Demon should not be a different character
Finn Balor (when he was known as Prince Devitt) first wore the body paint that has become a hallmark of his character at New Japan's WrestleKingom 8 event in January 2014. He would continue to wear the paint, in many, many different iterations (comic book characters, mostly, and other designs), for the rest of his time in New Japan and independent companies in the UK until he officially signed a contract with WWE in the summer of 2014.
During his time wearing the paint from January through June, before signing to be part of NXT, the paint was part of his character, but not a completely different character. He was a cocky guy who liked theatrics and being over-the-top. It was war paint, which also helped get him fired up and inspire him to do better and fight harder. The paint did not transform him into a totally different individual who was suddenly nearly invincible. It was a stylistic choice.
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Upon joining NXT, this continued. Finn wore the paint for big matches, and while he won 7 of his 8 matches in NXT in which he was painted up, he wasn't "The Demon". He was Finn Balor wearing an awesome costume, making a cool entrance, and intimidating his opponents by using psychology to get into their heads. However, upon his move up to WWE, the paint stopped being a simply thing he did and became a separate character. It was presented to the fans as an upgraded version of Finn. It was called Finn Balor's "dark place" by the voice of WWE, Michael Cole.
This distinction was a problem that confused many fans. Sometimes he could become The Demon, and sometimes he could be Finn Balor. It was almost literally stated by the commentary team that he was invincible as The Demon. It would make sense for Finn Balor to go to his "dark place" when he was faced with the challenge of The Beast, the undefeatable Brock Lesnar, but that did not happen, and fans did not understand why.
Because of the Lesnar match, as well as a number of other big matches in which he was Finn instead of The Demon, WWE may be better off no longer using The Demon as a separate character. Fans have called it confusing, and arenas have seemed to be disappointed when Finn's music would begin and The Demon was nowhere to be found. When Finn returns to battle Bray Wyatt and The Fiend, he will return as The Demon in order to even the playing field.
It may be in WWE's best interest to use that as an opportunity to end the use of The Demon as a totally different character. The Fiend defeats The Demon and Finn Balor is no longer able to get himself into his "dark place". He could still make the fans happy and play mind games by wearing the great-looking paint, but Finn Balor can be a stronger man without having to become The Demon.