Reverse Isekai! Looking for a caretaker for a warrior out of time (FxM, looking for M)

Name: Nimue
Race: Human
Sexuality: Straight
Height: 133 cm

-Background-
Older than she looks, Nimue has a bigger history behind her, resulting in quite a few things. A powerful arsenal, her having stopped aging, but also certain burdens she has to live with. Despite this, she is truly happy with how things are, for she was not always as free as she currently is.

-Personality-
Nimue is selfish by nature, and fully aware of it. Despite holding a lot of power for a human, she doesn't feel responsible to be a heroine at every opportunity. But she dislikes cruelty, suffering, and will use her resources to help if she sees a change to make things better. But while no crisis is in her immediate range of influence/attention, all she wants is good food, a warm bed, and free time to either indulge in her hobbies, or discover new ones.

The city park changed a lot over the years. It started as a few benches on a quiet street near a river, eventually got a bridge though, expanded, got shaped into what people would actually call a part, complete with trees, walkways, and shrubbery. The only downside was that the streetlights were constantly broken, and sooner or later, a few myths were made about it. Not that those who just wanted to take a shortcut thought it after a long shift would care though.

But whenever he wanted to or not, today was the day he would find out if anything about those occult rumors was true. Because as he made his way through, a peculiar distortion formed in the air. As if space was distorted, waved left and right. The hum of the breeze simply died, and got replaced by an otherworldly resonance that prickled at the edges of his senses.

Before he could fully grasp the situation, a shimmering portal materialized not far from him. Out of it tumbled a demon, its grotesque form contorted in pain, bleeding, and retreating by the looks of it. But it didn't come through the otherworldly gateway on its own. First a barrage of arrows, then a small figure, shrouded in red pink and black, came through. With the arrows now embedded into the demon as well, she pushed forward, grabbed her lance, forced it into the demon's back, pushed its mass through, and the demon with it.

The demon writhed and snarled, its guttural words in a foreign tongue reverberating through the air. The girl, undeterred, pressed forward, forcing the demon to lurch until it collided with a large tree in the park. The demon let out a final, eerie utterance, and suddenly, its once menacing form turned dark, then brittle. It shattered into chunks that dissolved into a fine dust, carried away by an ethereal breeze.

And just like that... it was calm again. One might call it a fever dream. But the girl was still there, exhausted, wounded, but... mesmerized too. She looked around, looked at the street lights that suddenly decided to work, the houses in the distance, the benches to her right. "Where... Where am I?" The surprises didn't end there. In shock, she let go of her lance, and it dissolved into shadow, then nothingness before hitting the ground. Suddenly, she noticed a presence, changed her stance, from golden light a bow appeared in her left hand, while an arrow did the same in her right. And while aiming at the person, she demanded "Show yourself!"