Both of the Four of Us
Prologue
The truth was, Luna craved adventure. She was not one of those ponies with an addiction to adrenaline; she had an addiction to importance. Life in Canterlot--or, to take a leap, anywhere in Equestria--didn't bore her by any means, but it did depress her. She may have been a princess, but she was not the princess, and even now she couldn't help but resent that her subjects respected Celestia so much and Luna so little. But, no, she couldn't think that. Not again.
This was why Luna needed to leave. She needed to go on an adventure, and not just any adventure. One where she was the heroine. She didn't have to be a princess as long as she was a heroine, and she was most certainly not one now. She would gladly give up her title for the chance to be an adventurer, to defy death and earn the love and respect of ponies the way no monarch could. Not even her sister. There she went again. She had to stop comparing herself to Celestia, or else Nightmare Moon would steal her sense again. Good luck earning ponies' respect after that debaucle happens again.
"Princess?" Luna raised an eyebrow, turning to face the royal guard who had addressed her. He was a bat-winged pegasus, one of her own, with a deep and sonorous voice. Immediately the alicorn adopted a countenance of unconcerned haughtiness, her head raised high into the air.
"Yes? What is it?" she asked sharply, eyes narrowed.
"Her Highness, Princess Celestia, would like to inquire if you are prepared to depart."
Yes. Luna's departure. For a moment the princess's mask of indifference slipped and she smiled, albeit only a bit. "Indeed we are," Luna replied. "Does my sister need anything before we leave?"
"No. She sent me to issue a farewell in her place and to assure you that she will take care of both the sun and moon while you are on her journey," The guard straightened his posture and looked into his employer's eyes. "Your guards bid you farewell as well, Your Highness."
Luna relaxed, again showing just the hint of a smile. "Indeed. Good-bye. Send that message to Celestia--and your fellow guards, if you will."
"Of course," The bat-winged pegasus bowed again before turning away and exiting Luna's study. The alicorn dropped her facade, relaxing and stretching her wings. The window behind her yawned open; usually it was shuttered to stave off the sunlight, but now it was opened wide in preparation for her flight. Honestly, she could simply leave through the castle entrance, but Luna loved showmanship as much as she loved adventure. So she leapt through the opening, the tips of her feathers brushing the sides of the window frame. She didn't like daytime, but she did appreciate the visibility it offered. She didn't dare look down; it was better simply to imagine the denizens of Canterlot on the lowly ground staring in awe at her beauty and grace. She hoped she held up hoof-traffic. Oh, she at least hoped that they were looking at her.
Luna flapped her great wings once, descending ever-so-slightly as she picked up speed. She angled herself to the south; that was approximately the way to Ponyville from Canterlot Castle. Still she didn't dare look down; instead she occupied herself by repeating the name of her soon-to-be traveling companion in her mind, committing it to memory.
Pinkamena Diane Pie. I do hope you are prepared to be a heroine as well.