Percy POV
I started walking to the the castle in the middle of the camp. The camp was massive, with dozens of cabins surrounding the gigantic castle in the middle. I entered the castle and stared around. It seemed to be even bigger on the inside, and I had to travel several flights of stairs before I got to the top floor. I opened the door and saw Harry with Chiron and a girl I didn't know.

"Percy! I would like you to meet Andromeda. While you have been going on several quests, so has she. Without her, we would have lost several times. She was once undercover at the school for magic, and sent a phoenix named Fawkes to save Harry."  Chiron stated.

"Once, when Thalia was captured and injured, she healed her, and that is why Annabeth remembered her eyes being green in your first adventure. She also stopped Leo from getting his body mutilated. If she hadn't, the physicians curse wouldn't have worked, and he would still be dead. She stayed at the Hermes cabin when she was at camp, and changed her name, and some of her features. These are only a few of her accomplishments that she has told me. Without her, this war wouldn't have been won."

(Just used the video above for some plot holes that she could solve.)

I looked at her. She had long black hair, and was very tan. She looked athletic, and I could tell she had been through a lot. 

"My mother was a water nymph, and I never knew my dad. I have some of my mother's powers. I can breathe underwater, and have some control over it." she explained.

I voiced the first thing that came to my mind. "My dads Poseidon,"

"And the naiads don't think highly of you because of that. Some of them respect you for beating Gaea, but others still think of you as 'that young upstart son of Poseidon'" she said, with a small smirk on her face. I could see she wasn't even trying to hide it.

I hadn't realized what effect I had on naiads. Come to think of it, I must have messed up thousands of them during battles where I used massive amounts of water. Every time I went swimming, I saw nymphs giving me dirty looks, and I always wondered why.

"Oh," I said.

Real smooth, Jackson, I thought.

"Andromeda will be showing you around," Chiron said.

The three of us walked out of the room.

"Sooo . . ." I said started, trying to make conversation.

"That's the castle, those are the cabins, and there's the arena. Tours over," she said quickly.

"Wait. At least show us where we will be staying." Harry said.

"Fine. You and the rest of the exchange students will be in the heroes cabins," she said, obviously bored. "I suppose you don't know where that is. Follow me,"

We trudged along with her to a cabin that had all of Olympian greek gods symbols, and a weird circle with a line that was in a triangle.

"The Deathly Hallows," Harry breathed out.

"So, Andromeda, you said your mom is a nymph, not a god, so how do you live here?" I questioned.

"I was raised by nymphs, but my mom and dad were a god and human. The Naiads told me about the Greek Gods, and the Wizarding World. They gave me their blessing, and told me that I was the champion of their kind. The nymphs are the ones who gave me my name. They also gave me this." she responded

She took off her necklace, and twisted a charm on it. It turned into a full sized gleaming trident, that gave off a strong aura of power. 

"It's not celestial bronze, stygian iron, imperial gold, or even stygian ice. It's some metal that we haven't seen before. It helps me harness my powers, and I'm stronger because of it. I still don't understand why I have these water powers. Sure, the nymphs blessed me, but they said you had to have 'the blood of the sea' for it to work." she explained.

"C'mon, there's still a few minutes before dinner, so let's go to the beach," I complained.

"Fine, but only because the director told me to hang out with you, so you didn't feel like a loser,"

With that, she started walking off to the beach, leaving me with my mouth open, and Harry laughing his head off.

"Got you there, mate." he said, following Andromeda.

****************************************Travel to Beach*******************************

"This actually helps me think," Harry said, while we were relaxing, and looking at the sunset.

"Yeah, it's almost like being at the sea,-" I started.

"Clears your thoughts? Yeah, I feel that too" Andromeda said.

"C'mon, lets swim!" I ordered, dragging both of them along.

We splashed into the water, and Andromeda and I had a massive water fight.

Just as we were laughing at Harry, an explosion of blue-green light fell around Andromeda.

"Andromeda, you have been claimed," a female voice rumbled.

Above her head was a dolphin jumping across an ocean. 

"Are you my sister?" I said, in shock of what had just happened.

"No, it wasn't a trident." she said. "Who is a god or goddess of water?"

We said it at the same time. "Amphitrite,"

"Wait, what? Does that mean that we're related, because Amphitrite is my step-mom, so does that make you the evil step-sister?" I mused.

"Percy, we don't need to listen to one of your weird rants right now," Harry said, with his british accent.

Ha, British accent. Styx, ADHD.

"What was that that thing?!?" he questioned.

"That means I was claimed. By Amphitrite. The wife of Poseidon. It could've been literally any other god. Literally any. I'm so dead."  Andromeda worried.

"Why?" I said, not really following the conversation.

"Percy, what if Hera cheated on Zeus?" Andromeda said.

"Well, he would probably kill the child, cause some huge storms, and try to find Hera." I answered.

"Think of what Poseidon could do to me, or the nymphs that I lived with!" she shouted.

"Oh, that would be bad." I said.

"Don't worry, I'll just have to hang out with you more. I mean, he won't kill his own son, right?" I said. 

When no one answered I said "RIGHT?"

"We've got to go tell McGonagall," Harry said.

"No, Chiron." I said.

"Chiron"

"McGonagall"

"CHIRON"

"MCGONAGALL"

"Quiet! We're going to tell the camp director." Andromeda said.

"Dionysus?" I questioned.

"Dorothy, you're not in Kansas anymore." she said, rushing us along to meet her or him.

"Ah, hello children. What can I do for you?" the lady in front of us said.

Her face was ageless, and she couldn't have been that old, I would say between 40 and 60. It was hard to tell.  She smiled at us sweetly, but I could tell she used to be a troublemaker, back when she was a kid.

"Oh, where are my manners? Do you want a cookie, young man?" she said.

"Yes, please." Harry and I said in unison.

She put down a plate full of them, and soon they were finished.

We all sat in the comfy chairs, and we were at the 5th floor of the castle.

"So what seems to be the problem?" she said.

With my mouth full of cookies I said, "Can I just say, that these are the best cookies ever?"

"Agreed," Harry said.

"I've been claimed." Andromeda deadpanned.

"Oh, wonderful! Who by?" she said.

"Amphitrite." Andromeda answered.

"Oh dear," the lady said, with a look that didn't exactly inspire confidence." We need to get you safe, and now."

With that, a bright flash of light appeared, and our molecules were ripped apart.

A/N

I read a Demigods Go To Hogwarts story by Claire_Valdez and it's amazing! You should check it out! She also has a demigods and Hunger Games Crossover, and I loved it! I just finished it today!