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The Closing Credits song "And Forever" by Robbie Danzie and Naoki Takao. It's amazingly heartwarming from an anime that seems to specialize in Mind Screw. Not to mention how cute Roger and Dorothy look sitting on that giant hourglass!
Doubly so when you realize the lyrics could be about Roger and Dorothy's relationship.
Dorothy actually hums the song throughout the series, further implying that she and Roger are the ones singing.