Related to Melodytale, an Undertale+Mermaid Melody AU, this is Deltapearls, using Deltarune. The music in the battle scene was Legendary Buster by Dash Track. Lyrics from Man on the Internet, albeit changed. Also, there are greyners. Based off the manga since Melodytale is based of the anime
Kris:(Narrating) Today is the opening day of the pearl baths. And It's gonna thrive thanks to us, the mother and son duo!
Kris: Thank you for coming.
(After the bath closes)
Toriel: Great work today, Kris. Shall we take a bath too?
Kris: Yeah, mom.
Kris:(Narrating) The truth is I'm actually a lightner.
?: hE is alWays doIng tHIs...
(Enter Temmie, the guardian of the two)
Temmie: kRIsP! cAnt yUO at LeaSt pUt your clOthEs iN tHe hAMpeR?!
(Kris splashes water on Temmie)
Kris: Kya! Pervert!
Temmie: wHaT aRE yOu dOIng?!
Kris: Sorry. I'll sing a sad song for you!
Temmie: Uh oh...
Kris:(Off-key) KIITE MUNE NO HAJIKETE TOBICHIRU NANDAROU!
Temmie: ReALLY! wHAT WeRE yOu ThINKiNG?! LiHgTnerS witHoUT PEARLS aRE cOMpLeTElY toNEDeF! doNt FoRGEt. wE cAMe to ThE WORLD OF GREY to lOOK foR yOuR PEARL! bUT thE rADeR is BroKen
Kris: Heh, your radar really sucks, Temmie.
Temmie: wEll eXCuSE mE! tHIs is ThE fIRST tiME tHaT iT'S hAd a StrONg reActIOn!
Toriel: Well whatever the case, it seems like seems like there's no mistake this time. First of all, a pearl is a Lightner's other half. You need it no matter what to become an adult Lightner. But that's enough. Go to sleep.
Kris: Okay...
(He left)
Temmie: Is iT oKAy? tO nOt tEll kRIsP thE tRUth? thE WORLD OF LIGHT haS bEEn acTIng StRangE RecEntlY sO tHe mAtTer Of kRIsP's esSentIal
Toriel: We'll tell him when the time comes. Kris is still a child after all.
(Meanwhile)
Kris: My pearl, huh. I wonder if she still has it. That was seven years ago.
(Flashback)
(Kris as a bright light watches a ship)
Kris: Wow! Is that what they call a ship?
(He started singing. A girl heard the voice.)
?: A singing voice. Where's it coming from?
(She saw Kris)
(A ten story tall wave crashed onto the ship)
Kris: The ship!
(He tried to wake the girl. He put his blue pearl, disguised as a pencil, on the girl. She woke up.)
?: You are...
Kris:(Narrating) The dream of then... my First love
(Flashback)
(The next day, Kris was walking on a pier)
Kris: The ocean in the morning feels Great!
(He saw a reindeer monster playing beach volleyball)
Kris: Wow!
(The monster, named Noelle, walked up to Kris. She reminded him of the girl he saved seven years ago)
Kris: Have you and I met before?
Noelle: Are you hitting on me?
Kris: That's not what I meant!
Kris:(Thinking) What was I thinking? That flaky girl is nothing like that girl.
Noelle: Well whatever you say. If you think I'm hot, come back this Sunday. There's a beach volleyball tournament that day. I'll be waiting for you.
(Noelle winked. Kris's heart pounded)
Kris: You're not hot at all!
(He ran away)
(Noelle reached for the pearl pencil the light gave him seven years ago)
Noelle: Nah it can't be.
(What no-one noticed was a goat with a green hat was watching)
(At school)
Alphys, the teacher: W-We got a new student today.
Kris: My name is Kris Dreemurr. Nice to meet you!
Alphys: Your seat is over there. You can look on the textbook of the person sitting next to you.
Kris: Ah! The girl from this morning!
Noelle: What a coincidence.
Kris: Yeah
(Several boys are chatting about Kris and Noelle)
Kris:(Narrating) Somehow... she seems familiar.
Noelle: You're going on Sunday, right?
Kris: Ah... umm...
Noelle: Of course you are
(The same goat from that morning came up to her)
Ralsei: Dreemurr-kun, do you know Noelle-san? I'm Ralsei Rumerred. Call me Ralsei
Kris:(Thinking) It's okay to call him that?
Kris: Oh, of course!
Ralsei: You're going to cheer on the beach volleyball match? Noelle-san's going to be so nervous!
Kris: I-It's not like that
Ralsei: Alright! I'm going to teach you Ralsei's secret girl getting formula! (Winks) After school, we'll go shopping and look for some swimsuits that'll really help you cheer her on.
(Ralsei and Kris took selfies, went out for ice cream, etc.)
(Timeskip to a swimsuit store)
Kris: This is so much fun!
Ralsei: Kris, I found a good swimsuit for you!
Kris: Oh, which one, which one?
(Ralsei shows Kris an incredibly revealing speedo)
Ralsei: This one!
Kris:(Panics) Gya! No way! I'm going to be seen by girls too aren't I?
Ralsei: Why of course! You'd wear this speedo for female eyes!
(Ralsei notices a Deltarune locket)
Ralsei: What a pretty pendant!
Toriel: Kris!
(Kris and Ralsei turned around)
Toriel: So you're loitering around here! Hurry up and get back to the bathhouse. I need your help.
Kris: I've completely forgotten?
Ralsei: Your mother?
Kris: Yeah. She's scary when she's angry. So I better go!
Ralsei: No problem. Bye!
(At the bathhouse, Temmie was shouting at Kris)
Temmie: whAt In tHe woRLd ArE yOU tHinKing, kRIsP! yOu wanT tO gO tO the bEaCh! iF YoU tRAnsfOrm wiThIn mUsiC, alL teh gReYnerS wiL PaNiC! i CheCkeD. ThE PearL isN't iN thE OcEAn!
Kris: I know that, but... It's the ocean, it's the ocean! Don't you want to come too, Grassy? I'll keep it a secret from my mother
(Temmie rages. But gives in)
(Sunday, at the beach)
Ralsei and Kris: Kya!
Temmie:(Thinking) I'm only studying the outside world. Studying!
(Ralsei runs to the water)
Ralsei: Kris, hurry up and get in!
Kris: B-But my bathing suit
Ralsei: It's alright! Don't worry about it!
(Ralsei opens Kris's jacket covering his swimsuit)
Kris: What are you doing!
(Glamour shot of Kris in his speedo)
(Some girls came up)
Girl 1: You guys are cute. Let's play
(One of them grabs Kris)
Kris: H-Hey! Stop!
(Noelle grabs the girl's arm)
Kris: Noelle!
Girl 1: The champion from this year!
Girl 2: Oh, is he yours, sorry!
Kris:(Thinking) Could it be that she was protecting me just now?
Noelle: You look cute in your bathing suit. But I like things a little more risque.
(Kris blushes)
Kris: You pervert!
Announcer: Would all entrants get ready please
Noelle: I'll show you how cool I can be.
(She looks at the ocean)
Noelle:(Thinking) The waves are rough today
(Noelle draws a heart with the pearl pencil and blows the paper for good luck)
(A darkner senses it)
?: I see it. I see it. The thing born from the light. The one that rules the light. I can't let it slip away
(Noelle's beach volleyball team were winning with 20-18)
Announcer: Amazing! The Reindeer, led by none other than Noelle Holiday, are truly amazing! That's Noelle for you. Awesome!
Kris:(Thinking) How cool!
(Noelle looks at the waves)
Noelle:(Thinking) The waves...
(The waves swallowed Noelle up)
Announcer: Looks like the waves swallowed her up. She's not coming up? Life guard
Kris: Noelle!
(Everyone ran)
Kris:(Thinking) This storm... It's just like the one from that time!
Ralsei: Kris, where are you going?
(Temmie looks at the pearl radar)
Temmie:(Thinking) What? The radars picking up the traces of two pearls.
(Kris ran into the water)
Kris:(Thinking) Nobody can see me here. It's safe
(He sang a short tune, turning him into light form)
Kris:(Thinking) Noelle, please be safe.
(A diamond shaped rock hit him)
Kris: What a strong rock!
(He heard a tune. He ran over to it)
Kris: There's a melody coming from over there!
(It was Noelle! Being held by a strange diamond snake)
Kris: Noelle! Who are you? Give her back!
Ruddin: So you're the owner of this pearl. I've been waiting for you. So I can make you and your companion mine!
(Ruddin throws diamonds at Kris)
Kris: What are you doing?
Kris:(Thinking) Ugh... I can't forgive him
(Noelle wakes up)
Noelle: This voice
(Noelle sees the light she saw seven years ago)
Noelle: The light from that time!
(She threw the pencil pearl at Kris)
Noelle: Catch!
Ruddin: What are you doing?
(Kris catches the pencil, which turns into a sword. He became coporeal, wearing a warrior suit. He can also, in his peripheral vision, see a vertical bar saying MP 0%)
Kris:(Thinking) Is this me?
(Some music plays)
(Kris wields the sword. He attacks the enemy, but it wasn't enough.)
Kris: How on earth do I defeat him?
?: Sing. Sing, warrior.
Kris: Who is that?
(Kris recognises the music playing. It was a battle remix of the Legend)
Kris:(Thinking) I can hear the melody again? I can sing! Wait for me, Noelle, because I'll save you no matter what.
Once, light and dark lived in peace
Then dark had shattered harmony
Brought a great calamity
To the dying earth of fear
Only then shining with hope
Shall appear the seven heroes
Seal the fountain, banish foes
And be reborn in the storm of the coast
Seven pearls of light bring the light of day
and face the light of true
Their eyes on the ball, injustice will fall
They'll defeat the dark all through
The seven pearls will travel side by side
Through the dark domain
The light of the rainbow together
All will stop the rain
(Kris saw that the MP levels were going up. The waves disappeared)
Ruddin: Damn you, you impudent little...
Kris:(Dealing the final blow) Courage Spin Pitch! How about an encore!
(Ruddin fled.)
Kris: He disappeared.
(Kris brought Noelle to some cape)
Kris: Noelle
(He noticed a pearl disguised as a pencil)
Kris: This pearl... You are...
(Noelle woke up)
Noelle: So it was you after all.
Kris: Oh!
Noelle: You saved me again
Kris: It was thanks to you, too
Kris:(Thinking) Noelle
Noelle: You were crying that time, too
Kris: I've found you at last!
Ralsei:(Off screen) Noelle?
(Ralsei saw Kris)
Ralsei: So It's you, Kris...
As always, let me know if you want more