"Don't worry, Yukkii... I will (ahn♡) protect you."
Yukiteru Amano and Yuno Gasai participated in a survival game created by Deus Ex Machina, the God of Time and Space. In this game, twelve participants use their respective "Future Diary" to eliminate each other and the last man standing will become the new God of Time and Space.
Right after Deus revealed the game to all the diary users and adjourned the meeting, Yuno stood behind Yukiteru and said the above phrase, making him realize he just entered a game that involves life and death.
This particular expression Yuno made became famous due to the impact it made on the viewers. It has since then become a parody in which many artists create their own versions of this pose using different characters.