9. Calendar Man
Batman villain Julian Day the Calendar Man is so called since he commits crimes on significant dates like Christmas, Halloween and St. Valentine's Day. For years, he was regarded as a silly but forgettable villain, which was all too common during the Silver Age.
But in the 1996-1997 miniseries Batman: The Long Halloween, Day was remodelled into a sinister serial killer. Regarded as a danger to society, Julian was incarcerated in Arkham Asylum and had become one of the few permanent residents of the facility. Even though this story helped Calendar Man become a more recognised name, nothing was really done with the character since.
When DC Comics rebooted under the New 52 banner, it was a perfect opportunity to revisit the character and make him more mainstream. Instead, he was turned into a comedic news anchor.
When DC again relaunched several years later, Day was completely revised as a metahuman whose age is based on the seasons of the year. On 31 December of each year, he dies and is reborn in a new body the following day. This is an intriguing idea but Julian Day is mostly absent since the relaunch, not giving the character a chance to be explored.