This isn't actually part of the challenge but I got mad at how challenges many were dedicated to ships (it was only three but still). So I put it in (which is why I didn't put 'Day' in the title).

And this day goes to Light Yagami from Death Note!

Death Note starts out with Light as an ace high-schooler getting ready for college. By the end of the show, Light is an adult and out of college, despite the show being 30 episodes long.

Unlike many MCs, who grow throughout the show and their peak is the climax, Light grows too quickly. His peak is far before the end of the show, even before the climax. After his peak, he begins to go downhill, forgetting things he has learned out of greed.

Still, if you plan to watch Death Note, don't get too disappointed. The change in MC characteristics from standard stories makes the story fascinating and certainly a binge watcher.

Honorable Mention:

Sadao Maou/Satan Jason [Hataraku Maou-Sama! (The Devil Is a Part-Timer!)]

(Credit to Hald)

Ritsuka Tachibana (Dance with Devils!)

Lol two and a half devil shows were picked for this day (if you count Ryuk from Death Note part devil)