I do not own Gravity Falls.
There was a strong connection between Mabel Pines and Bill Cipher.
Mabel's bright attitude, positive outlook, and trusting nature meshed perfectly with Bill's sarcastic leanings, dark perspective, and boarded heart meshed perfectly. They completed each other.
Usually, such a pair would be seen as a match made in heaven, clearly on the road to many happy years together.
Usually, such a pair would be completely human, and would not have any unforeseen consequences for not being so.
It had started slowly. Mabel seemed off in conversations, as if she wasn't completely there. Her brother, Dipper, even went to ask Bill if he had noticed.
Of course, he had. Bill noticed the slightest change.
The problem became more dire swiftly.
Bill walked in on Mabel trying to conduct a full conversation with her shadow. She didn't recognize her closest friends when they threw her a surprise birthday party. Her gaze was always distant, as if she was everywhere but where she was actually standing.
It took Bill far longer than he wanted to figure out the cause, to discover what was destroying his Shooting Star from the inside out. He didn't like the answer after his investigations finally came up with a result.
It was him.
"Wha-what's happening to me, Bill?" Mabel whimpered to him one night soon after he had uncovered the cause.
He felt his throat close up as he tried to answer his wife. How was he supposed to tell his true love, the only human that had ever broken his walls, that it was her contact with him that was making her sanity shatter?
He should have known that prolonged contact with a powerful demon of the mind would be harmful to a human. Even with his own human form, he would practically radiate power, which would only have brief effects on those he simply passed, but for one he spent almost every waking moment with...
Humans were so frail, it could damage her so easily...
"I'll... I'll fix everything. I promise," he eventually answered.
He shouldn't have had to fix it., he should have known their passion would be nothing but destructive for his beloved, and nipped it at the bud.
Of course, he didn't, because of his own selfish emotions...
Guilt wrenched through his stomach as she smiled hopefully at him, leaning onto his chest. Shaking, he gently kissed her forehead, for once praying that he could find a way to save Mabel.
It was his fault. He had to fix it.
He had promised.
Months later, his promise was officially broken.
Bill placed a hand on the thick glass, trying to will himself into the padded room, to will himself closer to her.
"Is there nothing you can do?" he whispered hoarsely to the doctor behind him, his voice pleading.
"I know this is troubling to you, Mr. Cipher," the man didn't even look up from his clipboard as he spoke. "However, I am afraid your wife's mind is lost. She barely recognizes anything of reality. You have been the only one she's responded to, and unfortunately that isn't enough to help her."
Bill fisted the hand resting on the glass.
"I broke her," he continued whispering, this time to himself, though the doctor still heard him.
"Sir, even if you had abused her, I highly doubt you could have done such damage-"
"I would never purposely hurt her!" Bill snarled, turning to the doctor with rage filled eyes. "And you know nothing of how I could accidentally damage the only one I ever truly loved.."
With a shaky breath, he looked back at Mabel.
He only wanted to be with her. His greatest wish was just to wrap his arms around her, to feel her soft curls on his face, to hear her loving voice...
Of course, that couldn't happen.
"I am so sorry, my beloved Shooting Star," he said softly, before vanishing from the room, much to the doctor's surprise and shock.
He should have known contact with a demon of the mind would hurt a frail human.
He would not make that mistake again.