Ceres Fauna was the first to die.

On Earth, the skies darkened as factories sputtered thick plumes of smog into the atmosphere. When the clouds choked out the sun, nature withered and died, and in its place sprouted mechanical plants and animals to feed a new generation. By the time the Sun heaved its last heavy breath, the citizens of Earth had long since left for distant planets, spreading themselves like a virus among the stars. In time, their new homes would suffer similar fates. As the last spot of nature vanished from the universe, Fauna felt the cries deep within her spirit. Yet ever the kind soul, even in her dying breath, she could not bear to admonish her children's children.
"Forgive them, they know not what they've done." She sighed. "Oh my little saplings, how you've grown!"
And so Mother Nature gave until she had no more left to give, and was remembered only in the folktales of the distant past - and time ticked on.

Nanashi Mumei and Mori Calliope were next.

Though her people had successfully outlived their first home, it had become increasingly difficult for them to find new places to go. As space itself expanded, humanity endeavored to follow. They stretched as far as they could, pushing their engines harder and harder with each and every passing millenium. Mori would complain about how they always tried to make her job harder and harder - trillions of lives, spread out across light-years and galaxies - and Mumei would laugh in response, proud of the ones that had created her as she continued to watch over them.
But as the last sun-forge lay dormant in a dark corner of the universe, it was clear that their time had come. Humanity had conquered planets and galaxies and believed time and space to one day be under their control, but the reality was that neither had ever truly been within reach. All the stars in all the skies shrank, drifting further and further away from each other until even the most advanced engines crafted by the most advanced societies failed to reach their closest neighbors. Eventually, even the mere idea of a world outside their own became unfathomable. There were no more galactic empires, no more solar republics - just the people in the here and now.
As the last chapter of the last book was written, the countless unfulfilled wishes of the universe vanished. Civilization collapsed, and as the last breath was drawn, the Reaper laid down her scythe. They had always been at odds, and yet at the end it seems that they were always meant to be together.
And so they stood there hand in hand, fading into the eternal darkness together - and time ticked on.

The last were Tsukumo Sana and Hakos Baelz.

As the universe grew through the eons, Sana and Baelz had felt their powers grow. Yet while their powers had never really opposed each other for the first few quadrillion years they had known each other, they knew that one day, they too would disappear together. After all, they had been born together and had been with each other as long as they knew. It only made sense.
In the end, Space was no match for Chaos, and after countless cycles of spread and decay, there was nothing left, and nothing left to fade. Although the universe expanded, its contents stayed the same. Eventually, what remained was spread so thin that there was almost nothing at all. By the time the last protons decayed, the three remaining beings decided, there would be no more words that they needed to share with each other.
They reminisced the past, the times spent with and among mortals. Although it had lasted but a fraction of their vast lives, they could all agree that it had surely been more interesting than the past few million years. The Warden, despite her reputation for strict adherence to time, had been more than willing to bring them back for quick jaunts - as long as they stayed out of time. After all, what had already happened had already happened, and would forever happen. Staring from afar, they bid goodbyes to their past - lives, friends, loves - and returned to an empty world.
And so they followed in the footsteps of their friends who had disappeared all those years ago, shrinking into the background of an empty universe - and time ticked on.

And on, and on.

Kronii had always enjoyed her time alone - but even she could admit that this was a bit... much. She had never spent this much time alone - in fact, she mused, it was quite likely that no one had ever or would ever spend this much time alone. Thinking back, it was likely the first time she had ever truly rested. For as long as she could remember, she had been watching Baelz and Sana scramble around, fighting over who was stronger. She could even remember the moment that Fauna - that loving kirin - had popped into existence, followed shortly by Mori and Mumei. As short as all their lives had been, it had kept her busy until now. She wondered if perhaps that pesky little time traveler had been correct - maybe meddling in time wasn't such a big deal after all? She shook her head. In all the time she had lived, she had only ever had one goal - to guard time. This was certainly beyond her.
... or was it? What was she guarding time for? Was she to drift endlessly through space, forever, waiting? Surely anyone who had wanted to meddle with time would have done so by now - she had long since surpassed any time within human imagination, the mere thought of a number that large would drive mortals insane. Besides, no one else had ever borrowed her watch, and she had long since pieced together a full timeline of Amelia's adventures. For a while, she sat and thought. She had witnessed the birth of all her friends and had told them their stories a million times. But what of her? What was her story? She had known her mission, but who had given it to her? There had been nobody to witness her birth or her growth - of all the supposed immortals she knew, she was the only one who had survived thus far. And so she thought back, attempting to recall further than she had ever considered possible - and time ticked on.

In the end, she realized, there had been nothing before her and there could be nothing after her. Her life, her powers, time itself, was truly infinite - she had existed before, and would continue to exist forever - but she had to confirm herself. She stepped into the past, her mind racing with a million questions, one above them all - what awaited her at the start? She saw the universe contract, the thrum of nature and civilization once again spreading, then shrinking back down to a single blue dot. In a moment, trillions of lives passed through her hands, coming back to life, then vanishing as if they had never existed. Then, as space shrank further and further into an infinitesimal point with no hint of any other being, she realized. This was her destiny. All those years - how could she not have known? When she could go no further, she paused, and took a deep breath. This was all that had been and forever would be - she was ready. As she stepped out, all she could picture were the worlds yet to be created, the lives yet to be lived. With a smile on her face, she thought, "I'll protect this all my life. I'll protect this forever."
Her memories, accumulated across an eternal lifetime, faded into a distant future not yet made. And so her last thoughts echoed inside her head - and time started ticking.