"The three lovely alicorn ladies (minus cadence).
Just thought I throw up some words about my ever developing headcanon today. So let's go left to right, shall we?
Celestia is the ruler of Equestria, a huge melting pot country that has self-proclaimed itself as the center of friendship and tolerance. Well it's about half right, but for the past one thousand and nine hundred or so years Celestia has been in charge.
As far as appearance goes Celestia is pretty frikkin' big. Twilight, an average sized mare, is sandwiched between two giants, as most alicorns as old as Celestia and Luna are. She's also pretty thick. With muscle that is, not too much fat. She is noticeably bulkier than her sister though. Her hair is constantly flowing because she likes it that way. It also smells like peaches and cream because that's her favorite shampoo.
Celestia is fairly concentrated on her appearance. He acts regally, is polite, but has a friendly humor and smiles often. There was a spanse of about two or so years after Nightmare Moon was banished that everypony on the country feared her. She then proceeded to do everything in her power to make that not a thing anymore. Now a days, everyoen pretty much loves Celestia unconditionally. She's kind, patient and will welcome anyone in need to her castle. She's also very open to new ideas when it comes to her political and economical standings. She has several representatives from different regions of the country who advise her and help to vote on new matters as time goes on. These representatives are elected my the citizens of the regions.
Aside from that Celestia is a very laid-back mare. when she's not doing royal things or in a meeting she likes to spend her free time relaxing and napping in the back gardens in Canterlot Castle. She does love cake, not just eating, but decorating too. She's not very good at it but it all tastes the same. She's an avid fan of poetry and also loves to watch boxing and jousting on television or live in the ring.
Twilight isn't much different from when she was a unicorn. She may have grown a few inches, and her hair is notably longer but aside from the weird wings, she's the same old nerdy Twily. She does however get to rule over her own country. Now mind you, I started this headcanon during the late second season, early third season of the show so I started this all before she was an alicorn, had her castle, and all that jazz. I'm keeping it this way. Thank you. Her country is small and farther north than Equestria. It is called the North Star Nation.
In addition to being a huge book worm and smarty pants, Twilight is now involved in international affairs too, as well as the affair of being in control of a large piece of land and the inhabitants of said land. She spent a good two or three decades training with Celestia in the ways of becoming a princess and dealing with conflicts and trades and what not. After that she 'moved' to the NSN.
Luna has been one of my favorite characters to work with. There's so much there to work with.
Luna is in essence a huge, lanky, clumsy, insecure teenager with super magic powers and rule over the land of Nocturnia. She's bigger than any of her subjects (and bat ponies are huge in comparison to regular ponies) and was absent from the throne for quite some time. Luna was sort of "given" Nocturnia as her 758th birthday present. And by that I mean, one day Luna left and started her own species of equine to live in the wilderness just north of Equestria's borders. Things went pretty well for about 90 ish years until the whole Nightmare Moon mishap and the almost throwing the entire world into an Eternal night thing. Celestia Banished her. Nocturnia was left all alone. They managed ok, electing leaders and such, but the rest of the world suddenly became very, very afraid of all bat ponies and shunned the country as a whole. This made the Bat ponies a little bitter. Especially when Luna returned. Even though she was no longer Nightmare Moon, none of Nocturnia's citizens knew who Luna was (as a pony, leader, dignitary, princess). They did not like her for abandoning them for the ways of madness. I will have more on what went down when she came back later but in short she was gradually accepted and works with the elected leader rather than being incharge of them.
As far as personality goes, Luna is actually quiet. She keeps to herself. The royal Canterlot voice is purely for when she goes to Equestria because she knows it bugs Celestia. Other than that Luna will stay in her room. She reads a lot and enjoys movies and video games. She gets what needs to be done as efficiently and quickly as possible in order to get back to her Pokemon games and to wearing big comfy hoodies. She is less concerned with her appearance than Tia is. Way back when, when she still lived Equestria all she ever did was worry about her appearance. She has humongous silly bat wings; skinny, gangly legs; fluffy ears; and buck tooth fangs. Back in the day Equestrians hated her because she looked to different. She would mask her appearance to look like the Luna on the show. But really she was a big dopey night pony. Her hair is also not that color. She makes it look that way via magics when in reality it is the pale blue it was when Twilight and the rest of the Mane Six defeated the curse and she broke free. It's also shorter and she rarely brushes it. This makes it look nice for the public and other rulers/dignitaries.
I think that about sums it up for now
Any questions? Feel free to ask."