Takes a Left Turn at Retarded Toward The End.
I'll be honest, I quite liked the first two thirds of Mirai Nikki. It had an interesting concept with the future diaries (Though the author was clearly having difficulty coming up with all the ideas for different diaries.) The series could shift it's tone nicely, going from being lighthearted to fucking terrifying at a rapid rate and some of the plot twists did actually surprise me.
The characters, the ones that matter anyway, are pretty interesting too. The cast is comprises of a colourful bunch of psychopaths (Some of them so colourful that they probably moved to Japan after Batman kicked their asses.) The other diary wielders can be badass (Yuno, Keigo, Minene) some can be tragic (Tsubaki, Reisuke) some can be hilarious (Yomotsu.) and some are...weird (Kamado).
But the main focus when it comes to characters are Yukki and Yuno. Yukki is...an interesting case. For the first half of the series he's an utter bitch like Shinji Ikari, but towards the end of the series he does become a much more badass character. So he has some interesting growth. But the reason everyone comes to look at this series, and the most memorable thing about it, is Yuno Gassai: The Yandere Queen. She manages to jump between being adorable, terrifying, badass, creepy, attractive and sublime at complete random. You never know just what she's going to do next, she's both an invaluable asset to Yukki in surviving the game but potentially the biggest threat to him due to her psychopathic nature and unhealthy obsession with him.
But this series is not without it's flaws, and its biggest one becomes apparent toward the series closing act. While I won't spoil anything I will say the series tries to become more complex than it actually is, introducing elements like omnipotence and time travel and alternate dimensions.
It's like watching an episode of Breaking Bad
and then all of a sudden Silver Surfer
shows up and tells Walt he needs to go back in time and sell drugs to dinosaurs.