This is a review of the entire series!
I debated between giving this 2 or 3 stars, and would lean more toward 2.5. There were a couple of things I really liked about it, but many more things I really didn't like about it.
High schooler Yukiteru Amano is a socially awkward loner who considers himself a bystander. He'd rather mind his own business instead of getting involved with other people, writing every mundane activity of his life in a cellphone diary. Secretly tormented by his solitude, he invents an imagery friend called Deus Ex Machina who claims to be the Lord of Time & Space. Taking pity on the lonesome Yuki, Deus grants his diary the ability to record future events. After learning that 11 other people with unique future diaries show up around Japan, Yuki learns that Deus is not a product of his imagination, and he has been selected to take part in a death game that involves fighting to become the next ruler of causality by outsmarting each other with their new prophetic devices. Luckily for Yuki, one of the death game's participants is his fellow classmate Yuno Gasai, the prettiest girl in his school who happens to love him so much that she would put her life on the line to protect him.
Speaking of Yuno, she's where most of the fun comes from. She's a severely unstable, possessive sociopath that will happily slaughter anyone and anything that get in the way of her selfish romantic fantasies. She's famous for inventing the infamous yandere trope (psycho/killer/stalker love interest) While she can technically be seen as the villain of the series and it's hard to fully sympathize with her because of how violent, impulsive and controlling she is, all of the other characters are so unlikable and bland that I often found myself rooting for her to just rip everyone else in the story to shreds.
Yuki is the main character, but he's such a whiny and idiotic pushover that I couldn't connect with the unhealthy relationship brewing between him and Yuno. I understand it's supposed to be a psychological thriller, but the story seemed to romanticize their relationship rather than portraying it for what it really was: predatory, abusive and horribly toxic. I would've rather had Yuno as the main protagonist and explore her tragic and demented backstory through much more intriguing characters to bounce off of rather than through a relationship with one of the lamest dudes in manga history.
The plot jumps around way too fast. None of the competitors have interesting personalities or motives so I didn't really feel any stakes or tension in the action. The only side character I somewhat liked is (funnily enough) another batshit crazy girl who plays the role of a cosplaying terrorist named Uryuu Minene. The dialogue could get really bad as the characters never reacted to death, betrayal or horrifying situations realistically. There's a point where a character discovers three dead bodies in his girlfriend's house, runs away in fear, then casually remains her partner for the rest of the series like nothing ever happened. There are many more scenes exactly like this where characters don't react and process things in remotely realistic or believable ways.
Some of the twists in the later parts of the series involving time and causality were admittedly cool, but they're introduced in a rushed and convoluted way which makes it feel messy overall. The action was good, there were a few emotional moments and the absolute chaotic nature of Yuno and Uryuu were exciting to watch unfold, but everything else about the characters, story and conflict of the narrative fall apart at the seams.