Stargirl led Nina into the elevator, her celestial presence casting an iridescent glow on the polished steel walls. As the elevator ascended, leaving behind the bustling floors of the citadel, Nina couldn't help but feel a sense of anticipation building within her.

Stargirl, with her ethereal grace, spoke as the elevator climbed higher. "Nina, welcome to the Harmony Keepers' citadel. This place was created as a sanctuary for all nerdcore artists, a haven where they can find inspiration, collaboration, and a sense of belonging."

Nina's gaze flitted between the numbers above the elevator door. "So, it's open to everyone?"

Stargirl nodded, her eyes holding a spark of wisdom. "Indeed, every floor of the citadel is accessible to anyone who seeks it, save for one. The top floor, level 20, is reserved for the Harmony Keepers and select nerdcore artists and their guests."

Nina couldn't help but be curious. "What about the floors with negative numbers? What are those?"

Stargirl sighed, her expression briefly exasperated. "Ah, those are the underground floors. Some call it 'lower nerdcore.' It's where a few artists choose to reside, away from the bustling activity above. It has its own unique charm, but I would advise you to steer clear of the very bottom floor, level -5."

Nina furrowed her brow. "Why's that?"

Stargirl's eyes held a hint of gravity. "The stories of what happens on that floor are...unsettling. It's best not to dwell on it. There are mysteries even I, as a Harmony Keeper, can't explain."

As the elevator reached its destination, the doors opened to reveal the pristine splendor of the top floor, level 20. It was a breathtaking sight—a realm of artistic wonder and celestial beauty.

Stargirl guided Nina through the top floor, showcasing its various facilities. They passed a spacious cafeteria where artists from different corners of the nerdcore world mingled, their creativity flowing like a river.

They moved on to a state-of-the-art studio, where the magic of music was born. It was a place where artists could collaborate, experiment, and create without limitations.

Stargirl led Nina to the bedroom quarters, where artists could find rest amidst the harmonious chaos of creativity. Each room was a haven, a space for artists to recharge and dream.

Finally, they arrived at the gym—a sprawling space filled with the hum of activity and the clinking of weights. Stargirl turned to Nina, her eyes reflecting determination.

"Nina, this gym will become your second home in the coming days. As you prepare for the Harmony Keeper Trials, it will be your sanctuary of strength and discipline."

Nina felt a surge of determination welling within her. She had dreamed of joining the Harmony Keepers, and now that dream was within reach. The trials would test her resolve, but she was ready to embrace the challenge.

As Stargirl continued to tour Nina around, the catgirl's thoughts swirled with determination and ambition. She had found a place where her passion for music and her desire to make a difference could flourish.

Internally, she vowed to train her hardest, to hone her skills, and to face the trials with unwavering determination. She would join the ranks of the Harmony Keepers and become a guardian of the cosmic harmony, just like Stargirl.

With each step she took in the citadel, Nina felt herself drawn deeper into a world where destiny and desire were intertwined. The echoes of her ambition resonated through the corridors, a promise of a future where her dreams could take flight.

As the days passed and the citadel became her home, Nina knew that her journey had only just begun. With each sunrise, she grew one step closer to the trials that would shape her destiny, and the allure of becoming a Harmony Keeper filled her heart with purpose and passion.