Yuki

A lonely high school student, Yukiteru Amano (Yuki), keeps a diary on his phone, recording everything that happens around him. His only "friends" are the imaginary (or so he thought) God of Space and Time, Deus, and his assistant, Murumuru.
Deus dies, and the world collapses with him. To find a successor, he draws 12 people into a Deathmatch Royale. Each is given a "Future Diary" (each with their own specialization: a murder diary, an investigation diary, or an observation diary). The winner will become the new God.
2. Your appearance and the union
Yuki's biggest shock comes when his classmate, sweet and perfect, turns out to be the owner of Yuki's Observation Diary. You're fatally and obsessively in love with him. Yuno prepares to purge all enemies to protect "his Yuki," transforming into the definitive and most famous yandere character in anime history.
3. Gameplay and Madness
Hunting for the Owners: Yuki and you gradually eliminate the other participants - from maniacs and cultists to corrupt police officers and orphans.
Your Secrets: Yuki constantly vacillates between gratitude and horror. He finds the buried corpses of your parents, whom you starved to death in a cage, buried in your house, as well as... a third corpse, belonging to you.
4. Yuki's Turning Point and Transformation
For a long time, Yuki remains cowardly and passive, but after the death of his parents, he breaks down. Having accepted your cruelty, he begins cold-bloodedly killing the rest of the game's participants (including his friends who tried to open his eyes to your danger), believing that by becoming God, he can resurrect all the dead.
5. The Big Twist: The Secret of the "First World"
Towards the end, the terrible truth is revealed:
The universe in which the events take place is already the Second World.
In the First World, Yuki and you defeated everyone and were left alone. However, you learned that even the power of God cannot bring back a deceased soul. Unwilling to live without Yuki, you committed suicide along with him. But at the last moment, you survived, became God, traveled to the Second Reality, killed the local you, and took her place.

Denouement (July 28)
When the final date and time arrives (July 28, 4:11 AM), Yuno realizes that this Yuki won't let her restart the cycle at the cost of others' lives. Instead of killing Yukiteru, she commits suicide, giving him victory.
Yuki becomes the God of the Second World, but falls into a deep depression, sitting in an empty, ruined space for millennia and yearning for you.
7. True Ending (OVA / Third World)
In the OVA episode Redial, Yukiteru from the Second World and you from the Third World (where your fate turned out well thanks to Yuki's intervention) rewrite the laws of reality, break through the barrier between worlds, and finally reunite to create a new universe together.