I had this pop up a little while ago and I wanted to get it down. If you have any requests or headcanons, lemme know and I'll write 'em down for ya. They'll be added onto this as a "Headcanon Collection"

Estelle Blofis was three years old and Percy couldn't be happier. She could talk pretty well and was walking a bit. Like every normal baby does. But he hadn't accounted for one thing. One major thing.

It all started when Percy was in his room at his mom's apartment. Nico had shadow-traveled in to talk to him about something important. Chiron's 4,695th birthday was coming up and the senior campers wanted to do something special. As they were talking, Estelle wandered into Percy's room and saw Nico.

"Pewcy? Who tat?" Percy jumped and so did Nico. The latter quickly leaped into a shadow and was gone.

"No one, Esty."

"But why he go in dark spot?" the baby asked, pointing to the shadow. Percy frowned. She shouldn't have seen that. The Mist should have covered it up. Unless… An idea formed in Percy's mind. It wasn't good. Quickly, Percy picked her up and ran out to were his mom was making dinner.

"Mom, I'm taking Estelle up to the roof."

"What? Why?" Percy told her what just happened. Sally's eyes widened. She nodded and Percy sprinted out. He raced up the steps to the roof. Out of breath, he sa down for a second. Estelle tottered off.

"Esty! Come here!" Percy said. Estelle walked back to her brother. Percy held her hand. He looked into the sky and whistled.

"Blackjack!" he called aloud and with his mind. A moment passed until a dark shape hurtled onto the roof.

Heya, boss. What's up?"

"Act normal."

Whoa, hol' up. I smell mortal.

"Yeah, this is my sister Estelle."

Kinda small.

"Horsie!" Estelle grabbed for Blackjack's nose, but he jerked back.

Grabby, too.

"I have a hunch. Can you do a few tricks?"

Boss, you've seen me do some before.

"Not for me, for her." Percy nodded to the three year old. He felt the pegasus' surprise.

Is she, y'know…

"Don't know." Percy said. Blackjack took off and did some loops and corkscrews.

"Horsie birdie!" the girl squealed. Blackjack landed.

You hear that?

"Yeah." Percy picked up Estelle again and took her back to the apartment.

"Mom!" Percy yelled.

"In the kitchen!" He set the baby down and led her into the kitchen.

"Mom, there's something with Estelle." Those are words no mother ever wants to hear. Sally turned away from the meal she was making and looked at her son in concern.

"What do you mean?"

"Okay, so Nico showed up to talk to me, right?"

"So that's who I heard."

"Yeah. And Estelle walked in. Nico shadow-traveled away, but then Esty asked 'why he go in dark spot?' She saw him shadow-travel. So then I took her up to the roof and called Blackjack. She said 'horsie' when she first saw him, but then I asked him to fly."

"And?"

"She said, 'horsie birdie'." Sally's eyes widened. "Mom, I think Estelle can see through the Mist."