Some of the obvious ideas-

Princess Luna - fell in love withe Luna the second she saw her and has been a devoted follower every since, going as far as to join the night guard just to be near her idol

Night Guard/bat pony - The second most goth of all ponies.

Maud Pie - Moonlight Raven is a bit lively for Maud, but is the only pony with nearly as few emotions. Both enjoy poetry and the moon (which is technically a rock) so there's that.

Pinkie - Moonlight reminds her of her sister, and they say opposites attract.

A bat pony paring would be really cute!

4707263 I'd say she works for Luna's court! She seems like the type and opens up shipping doors for batponies and Luna.

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Sombra, who she trapped and reformed as Princess Luna's student.

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There's a pony/human named Marco DaFoy that I think would fit her well. Or maybe a changeling! Preferably or that was green.

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What was fucking stated? One is goth, one's super happy. They're sisters. Anything else is game at this point.

4707263 At the moment nopony. I really haven't had a chance to think about it yet.

This however,

Pinkie - Moonlight reminds her of her sister, and they say opposites attract.

is a very intriguing idea. I'd like to see that. Maybe even with a bat pony Pinkie as well.

Well, there's Pixel Pizazz. She's one of the background humans in the EqG movies and has a gothic lolita style. Here's the ponified version of her:

4707263 Luna because I see her as a fan of Luna and the Night I bet Revan would support Nightmare Moon