Saekina had a so-called life to disappear from, I'm not sure what this means without context and not enough time to wash the grime from the night before away. I think the first line could be stronger if it were more specific. What exactly does is Saekina doing to try to disappear? Maybe something like: Saekina ducked into an alley, but the narrow passage would only hide her temporarily. She needed to disappear, or at least wash away the night's grime before the rising sun left no shadows to shield her. The authorities would
try to arrest her, thinking she’d killed those slavers, and what would she say? “Sorry sir, some UpperSpawn, or at least, I’m guessing it’s a demon from the UpperWorld, broke in and killed those people and I’m the only one who lived? And I most certainly did not summon anything from the UpperWorld.”
Yeah, they’ll take my daggers and throw me in a cell.
Her back was throbbing, more out of discomfort than pain. The stone ground had not been the most pleasant way to spend the night, but she and alleys tended to find each other. Oh, did she just wake up in the alley? You may want to clarify that in the first line then. Waking up is generally considered a cliche opening, but I would consider waking up in an alley to be a fresh way of turning that cliche on its head. :)
She worked her hands up the wall, using it as a crutch until she was standing. Pain spiked through her head, followed by a burst of wooziness. She sucked in a breath and let it pass. She didn’t have time to feel sick. She needed to move. Her cheek had left an angry trail of crusty blood across the light brown skin I know this is third person, but it feels like limited third, which should have a deep POV. In that case, it would be odd for her to think of the color of her own arm/hand. Blending into crowds would be an issue. She scowled and wiped it off on her sleeve. Gods, she must look like a mess. A bloody, bruised, mess. I suggest rearranging the sentences as such for a more logical progression: While she slept, her cheek had left an angry trail of crusty blood on her arm. She scowled and wiped it on her sleeve. God, she must look like a mess. A bloody, bruised mess. Blending into crowds would be an issue.
The sour smell was the perfect way to top off what had been a supremely shitty night. She slid a palm over her face, dark brown hair collecting between her fingers. I'm picturing this scene and thinking she might notice the smell and wipe her face earlier when she first wakes, before she stands up.
While I think this is an intriguing opening overall, I do wonder if the night before would be more intriguing? Unless nights like that are the norm for your MC. If waking up like this is meant to give the reader a glimpse of your character's everyday existence then I would say this is a great starting point. Hope this helps. :)