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luxaofhesperides · 10 months ago

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Human/Mer AU + Bioluminescent Siren Duke ; requested by @justwannabecat!

The human hadn’t been in his territory recently.

He wasn’t attached or anything, but Duke had started to look forward to seeing him around. It was equal parts exciting and terrifying to be under the curious, watchful eyes of a human who could so easily be dragged down into the depths.

He knows he shouldn’t be doing this. He shouldn’t be indulging this human’s curiosity, trying to lure him closer time after time. Holding the attention of any human is dangerous; Duke’s heard the stories plenty of time. He’s seen the damage humans can do even more.

There’s just something different about this human, who never dives too deep, who smiles at him and leaves little gifts in the tide pools tucked away from the rest of the beach, hidden from sight. Or rather, usually hidden from sight, since this human is the only one to go there.

It’s foolish, but Duke actually misses his human.

It’s not the first time he’d been gone for long stretches of time, but something feels off. There’s worry curling up at the base of his throat, making him swim to the surface more frequently. Steph had given him a look when she caught him, but didn’t say a word. She shouldn’t, really, when she’s been sneaking up to the surface for her own human friend, some small, dangerous human with dark hair and hands that speak more than a voice.

He’d seen her, just once, when he had gone up to splash water and his human then swim away.

Somehow, things felt easier back then. Like the horrors of the world couldn’t reach them among those tide pools.

It’s reached them now.

Duke’s not expecting to see his human when he swims up to the surface. He’s expecting another quiet night, an empty beach, a dark sky with only the moon casting its lonely light down onto him.

But when he swims up, his eyes go to a figure on the beach instantly.

Even from this distance, Duke knows: that’s his human.

He doesn’t think before he’s swimming over, pushing himself faster than he’s ever gone before. It’s low tide, so he can’t get as close as he wants and can’t reach most of the tide pools at all, but it gets him close enough. Human and merfolk vocal chords are different; he can speak in water, but can’t make more than a few hums in air, and humans can’t really do anything in water at all.

His human is sitting with his knees tucked into his chest on the beach. He’s hiding in his clothes, a hood pulled over his head, but he looks up when Duke drags himself onto the sand.

Duke can see bruises. Dried blood. A stray tear slipping out of his eyes.

He wants to ask what’s happened? But all that comes out is a low crooning noise.

His human laughs, a quiet, bitter noise that makes Duke’s chest tighten uncomfortably. “Hey,” he rasps in a low voice. “Been a while, hasn’t it? I hope you’ve had a better time than I did.”

Duke can’t reach his human. The distance between them isn’t great, but it’s too much. He’s already partially out of the water, hands sinking into the wet sand just out of reach of the waves, and he can’t get any farther out. He reaches a hand out, silently pleading for his human to come closer.

The move makes his human soften, some of the hard edge of tension in his body melt away. He gets up and walks into the water, then sits down next to Duke, taking his hand.

“I missed you,” he whispers.

If they were underwater, Duke would be able to say I missed you too. Don’t ever go away so long again. But his human is in no shape to go underwater right now, so Duke presses his hand against his lips and hums lightly.

They sit in silence for a moment, and Duke realizes that he’s never been this close to any human before. It doesn’t feel dangerous. It feels like relief, to finally have his human in his reach, safe from the rest of the world.

He gives him human another moment, then reaches out and carefully pushes his hood back. His human allows it, blinking at him slowly. Without the shadow of the hood, Duke can clearly see the bruise coloring his cheekbone and the cuts going down his temple to his jaw. His split lip is still red with blood, and what little of his throat isn’t hidden by his clothes reveals more bruises wrapped around the delicate column of his neck.

Duke ghosts his fingers over each of these injuries, hating how easily humans hurt each other. His human leans into the touch despite how it must hurt, something devastating in his expression.

Who hurt you comes out as a questioning trill. Somehow, it gets the point across.

“It’s alright,” his human says. “Really. I’m not even that hurt. It’s just been a long few months. We never talk much, so you wouldn’t know this, but I have to fight a lot of people. Perils of being a hero, you know?”

Duke knows about heroes. More specifically, he knows about mer heroes. He’s considered being one himself, but the currents shifted and he ended up more a loner, banding with the other rejects of the city to live in the fringes and help only those who wander out too far from the marginally safer waters within.

He hasn’t heard of any human heroes, but then again, he doesn’t know much about humans at all. Nothing beyond the stories all parents tell their children to scare them away from the surface, or the horror stories kids tell each other in the middle of the night when they want to scare each other.

He hums again to let his human know he’s listening. His human has such a nice voice. Why haven’t they done this before?

It’s always been a push and pull between them, carefully keeping their distance but always circling back to each other. Duke would let his human swim with him, and his human would let Duke sit safely on the other side of a tide pool, tossing sea shells back and forth between each other.

They don’t even know each other’s names.

He wishes, just for a moment, that he could go back in time and do things better. But he’s happy here with his human and he doesn’t want to lose this either.

He’ll just have to make the best of what he has. It’s how he’s always lived after his parents disappeared.

“This really isn’t that bad,” his human says, “I’ve taken worse hits before. It’s just that I couldn’t transform before the attack started, so now my human form is bruised too.”

…Human form? The more Duke hears, the more questions he has.

Duke hums at a lower octave, placing a hand over his human’s chest.

His human laughs lightly. “Yeah, I guess we’ve never really talked much about ourselves, did we? I’m human, don’t worry, just not all the time. I… actually, I died a few years ago. But I came back partially. So I’m also half dead still and I can transform into a ghost to fight threats. I’m a hero called Phantom. Actually, Danny Phantom since I was stupid enough to just give out my first name when I started out. In my defense, my brain was still a little fried.”

There is so much he wants to say to that. He tries, and makes a series of low hums and clicks in the back of his throat, staring at Danny (he finally got his human’s name!) incredulously.

“I promise I’m fine,” Danny continues. “It was just a bit rough. As soon as I get some time to recover, I’ll be good as new! And I really did miss you, you know. Didn’t even go home first, just come straight here.”

That’s honestly really sweet. Duke hums again, a lighter pitch, and takes hold of Danny’s wrist and tugs him towards deeper waters.

“What? You want me to go in?”

Duke nods, already shuffling his way back out of the sand.

He expects to look awkward during the process. What he most definitely doesn’t expect is for Danny to easily pick him up and walk them both into the ocean.

Listen. Duke is not a small mer. He’s big. He’s got a long, heavy tail and wide fins going down his back, his forearms, and the sides of his tail. It’s a struggle for him to fit into seaweed nests with his friends during the colder seasons, often left to balance on the edge with his tail hanging out. His friends struggle to pull him through the water with his weight. His parents weren’t able to hold him much after he started growing.

None of this matters to Danny, who acts as if Duke weights nothing at all.

To his great embarrassment, his fins flare in appreciation for Danny’s strength. He was not expecting a human to be so strong, but Duke’s not about to lie to himself and say it’s not attractive.

He trills to Danny, who laughs again, then falls into the water, taking them both under.

Duke doesn’t hesitate. He grabs hold of Danny and swims them further out. He stays close to the surface so Danny can rise for air as needed, but he makes no move to leave Duke’s side even after a few minutes.

He glances back, concerned, when he sees that Danny is watching him with dark eyes, not breathing at all despite being conscious.

Danny holds up a finger and closes his eyes. Two rings of light appear around his waist, then split apart and pass over his body. Instantly, Danny’s body becomes lighter, as if Duke’s hand is grasping at a current on the seabed. His hair turns white and his eyes glow from how bright of a green they are, but there’s something inhuman about his features now, something that makes him look different beyond just physical features.

Well. Danny did say he could transform.

Now that they’re underwater, Duke can finally speak. The first thing he asks is, “What was that?”

Danny grins at him. “That,” he says, speaking with ease as if he’s not underwater at all, “was my transformation. And this is my ghost form. Quite the look, isn’t it?”

“You can talk down here?! Also, hi, my name’s Duke. Since I never introduced myself.”

“Ghosts can do a lot of things,” Danny answers with a cheeky grin. “It’s nice to finally know your name. I’ve been calling you glowfish in my head this whole time.”

“Glowfish?”

“Yeah. Because you, you know,” Danny gestures vaguely at him, “Glow.”

Duke glances down at his back fins, which are indeed glowing. They flare a bit from embarrassment, which just makes the dots of light lining the fins more visible. He doesn’t glow a lot, keeping a tight hold of that ability, but sometimes it comes out anyways when he’s feeling especially happy.

And apparently, he’s always very happy around Danny.

It’s a good thing Danny doesn’t know what that means, because if anyone of his friends found out, they’d laugh at how obvious he’s being. Drawbacks of being somewhat bioluminescent: anyone familiar with mers who have this feature know exactly what the glow means.

“Right,” he says just a beat too late. “Well, now that we know each other’s names, can you tell me how you really feel? Those injuries didn’t look too good.”

“It’s fine, really! I take harder hits all the time.”

“That doesn’t mean it doesn’t hurt.”

Danny doesn’t answer for a long moment, then sighs. “Yeah. It still hurts.”

“Stay with me for tonight,” he says. “You’ll be safe. You can rest and heal and I’ll keep you safe from anything that comes looking for you.”

“You don’t have to—”

“I want to.”

Danny doesn’t put up much of a fight. He must be exhausted. “Yeah, alright. Take me away, Duke. You know, this is like those stories about sirens luring sailors down into the depths.”

“Well, I am a siren.”

“Wait, really?”

“Yeah. I take after my mom. She’s the one who taught me how to sing.”

“I guess it’s a good thing we’re friends so I don’t have to worry about being dragged down to my watery death.”

Duke snorts. “Good thing you’re already a ghost then. Not much I can do to you down here.”

He swims down, heading towards a small cavern in a sea rock that he’s claimed as his own, leading Danny into it. The light from his fins illuminates the entrance and the rocks within, a narrow passage that goes in for a few meters before opening up into a larger space full of carefully tended to seaweed and starfish decorating the walls.

There’s a nook tucked away in the back wall where he’s set up a seaweed bed, the plant braided together into something more solid. It’s big enough to fit his tail, which means it’s big enough for him and Danny.

“Here,” he says, helping Danny down. “Get some sleep. Then you can tell me about what happened in the morning and we’ll take it from there.”

“I’m glad you’re here Duke,” Danny whispers, curling up on his side. He holds Duke’s hand, twining their fingers together, and it’s as nice as it is strange to feel how cold Danny is in this form when he was so warm as a human.

“I’ll always be here for you. You just need to come back to me.”

Danny hums, but doesn’t answer. It’s alright; Duke’s used to his loved ones leaving. He knows he can’t make them stay. All he can do is hope they return one day.

It’s been a long time since he’s had anyone in his home. There’s a communal cave where his friends stay that he visits when he gets lonely, but this place used to be for his family. Now it’s just him.

Him and Danny.

The last time there was song in these walls, his mother was still around, singing him lullabies.

Looking down at Danny, curled up and so strangely fragile looking, Duke feels the song build up in his chest. It slips out in low, soft notes, an old melody passed down through generations of their family.

He sings Danny to sleep.

He sings and sings and sings until all his nightmares are soothed and dawn is almost upon them.

It’s all he can do, so Duke sings and hopes it’s enough to keep Danny close to him for just a little longer.