Endless Void

Rarity awoke in her bed at home.
She had no memory of getting here, not even the plane trip home. But when she looked at the new outfits and saddlebags she had bought, there were more than she remembered.

She didn't really care, regardless. The clothes didn't have much meaning, and besides, there was something else she had interest in.
The mirror had no abnormalities. No warping. Her reflection was completely normal. She knocked on the glass, saying 'Hello?'
She felt stupid talking to her reflection. But then she remembered something the old colt had mentioned, as well as the copypasta.
Rarity looked at the mirror, and asked it, 'Friend?'
She went to knock on the mirror again, but her reflection stayed in the same position, then backed away slightly from the mirror.
Rarity was confused and shocked, even though she knew what to expect. She stumbled back and tried to make words, while the pony in the mirror watched her with something that was almost amusement.
'Something wrong, Rarity?'
Rarity tripped over her words, trying to come up with something that made fuck-all sense.
'I-uh-I-I gue-guess you're the Friend, then?'
Immediately her reflection answered 'Yes.'
They looked at each other for a minute or two, and when Rarity had calmed down enough she asked, 'So, what are you?'
The reflection replied, 'I'm your omniscient Friend, I have to answer every question you ask me with the truth.'
Rarity sat on the bed, thinking this over.
'So,' she began, 'You're like the projections in Inception? You're not actually real, you're just put out in my vision by some force?'
'No. Not at all.'
'So, you're real, then?'
'I was once. I may be still.'
'Okay. Then who are you?'
The reflection hesitated, then said thoughtfully, 'I don't know.'