You and her had meet before, You saved her from a light bulb falling on her head, She had introduced herself with the name Waifu because you're otaku and she heard that otakus likes to call girls as Waifu.

Now she destroyed the world with your help and You two are observating while meteors fall on earth destroying everything. "Finally, The World is Destroyed!" She's so happy that the world is ending, so you take the opportunity from the situation to confess your feelings for her.