I'm the head court jester and what I say goes

Foal Celestia:

When she was young, Celestia put her all into learning everything she could from her and Luna's father about her duties taking care of the day cycles and running the kingdom. She absolutely had a superiority complex, especially because there was more pressure to succeed put on her as the older of the two sisters. Despite this, she had a good relationship with Luna and spent what little free time she had playing with her.

After the passing of their parents, however, she spent less and less time with Luna. The stress of running Equestria mostly on her own kept piling on until she blew up, snapping at Luna that she should be pulling more of her own weight.

After the major falling out between the sisters, Luna would disappear for days on end and Celestia couldn't find the energy to figure out where her sister was going. It wasn't until after Luna found the Draconquis relic that it really set in what she had caused.

- 3 years older than Luna

- Resting bitch face

- Does most of the decision making and talking for both herself and Luna

- Generally stressed out at all times after the passing of their parents but does her best to hide it.

- Tries to rely on Starswirl as little as possible

The Banishment Era:

A long and painful battle between the sisters lead to the mangling of many of Celestia's feathers and the eventual severing of half of her tail, at which point she turned to the elements of harmony for assistance.

The elements were easily able to defeat Luna but ultimately did irreversible damage to Celestia as well, having forced her to grow into her full size rapidly in order to be able to handle the magic they provided.

The strain on her body made simply existing difficult for many years, chronic pains and difficulty keeping weight on lead her to withdraw inwards. The problem only grew worse after the hope and subsequent betrayal she received from Sunset Shimmer, to the point where her public appearances became even more rare.

Often, the only time you would see Celestia was if you caught a glimpse of her raising and lowering the sun far in the distance.

It took many years for her to take her next student, Twilight Sparkle, under her wing. At first, it was simply out of obligation; after all, a unicorn that powerful could turn out incredibly dangerous if left unmanaged.

Despite her best efforts to stay neutral and detached, she grew closer to Twilight as the years went by, put more and more effort into mentoring her, and found purpose in watching her grow.

Much in the same way as a mother raising her child.

The thought struck her with a lingering dread- one that only grew stronger as the end of the millennia approached- and drove her to prepare for the upcoming return of her sister.

Celestia, The Solitary Sunbeam:

- Colors have become darker

- Cutie mark has started to expand, the waves fluctuating in shade to through her highs and lows, never growing lighter than what is pictured in the art.

- Gained a Hannya motif- pictured through the eye markings and fangs, to be specific.

- Constant chronic pain, always thin enough to just barely be able to see her ribs.

- Very niche song reference in her cutie mark

Luna's Return:

If asked, she'd say her decision to send Twilight to Ponyville was a selfish one, one made out of an attempt to handle everything on her own yet again. Above all that selfish instinct, though, was a desperate decision to keep Twilight from seeing the fight Celestia thought would be her end and to force her to find a new support system.

When the night of Luna's return finally came, Celestia waited in her throne room only to realize with horror that her sister had somehow found out about Twilight and make her way to Ponyville. By the time she reached the town, the girls were already making their way to the old castle to defeat Luna. After the second defeat of Luna, Celestia met the girls at the edge of town. The sight of her sister among the six friends was a shock to her system, leaving her frozen in place until Luna approached her and she crumpled before her sister. Her voice broke with each apology, breaths coming in short and shaking between them. Only when Luna softly accepted did she allow herself to cry, tucking her sister under her wings and holding her close.

Over the years after Luna's return, Celestia's colors slowly grew lighter and her cutie mark finally fully grew out and changed into the color that it was always supposed to be.

- Much more outgoing again, going to as many events as her chronic pains allow

- Much fewer days where her pain is fully debilitating with Luna sharing the workload of running Equestia, allowing Celestia to take it easier and give herself the rest she needs.

- Regularly visits Twilight