Alright, the story:
Aqua stayed with the Doctor for as long as she could remember. He never let her out of the Tardis, but she had free range. So she usually went to the kitchen.
Where the cookies are.
Aqua hid behind the control panel thingy, making sure the Doctor couldn't see her.
He was talking to a orange-haired woman, who was smiling and laughing.
Aqua slinked into the hallway, and then into another room.
On top of the table was a big case of Oreo cookies.
Aqua sniffed the air, for no reason really, and then hopped up onto a chair. She started to eat the cookies, and forgot to watch out for the Doctor.
The lights in the room switched on, and Aqua froze, her ears sticking straight up from her head.
"Aqua!" the Doctor hollered. "You can't eat all the oreos!"
"Uh...whoops?" she tried, but he frowned. That red-haired woman was standing next to him, a smile on her face Aqua didn't recognize.
"She's so cute!" the lady picked Aqua up by her front legs, and cradled her like a baby. "What's your name?"
"Her name is Aqua, and she's a little too old to be stealing oreos," the Doctor said.
"Old? She doesn't even look a year old!"
"Donna, she's 1000 years old."

I'm guessing you guys want an explanation for her age. Well first, time-travel is confusing, so technically she could be that old because she's never left the Tardis, but remember in the last pic i said her kind could live up to four thousand years? When the Doctor found her, she was like five hundred, and she was still small and considered a dragonet. Okay, done with the confusing stuff, onto the sad part.

I forgot to add that Aqua can't age or get bigger inside the Tardis(huge time-lapse) One day when Aqua came out of her room, she noticed Donna was gone.
"Doctor?" she asked. "Where's Donna?"
He glanced at her, and smiled sadly. "Donna can't come back."
Aqua nuzzled his hand. "I'm sorry."
(another time-lapse)
Late at night one day, the Doctor came into her room.
But he looked...different. Aqua couldn't quite see him in the dark, but he walked differenntly, and his silhouette looked strange.
But he smelt like the Doctor, so she decided he was having a ruff night or something.
He kneeled down close to hee. "I'm sorry. I'm so so sorry."
He put his hands on either side of her head, and Aqua blakced out.
The Doctor scooped up the little dragon, and went out of the Tardis.
It was raining, so he had to be fast or the water would wake her up. He ran through a huge gate, with greek fire burning on each of its poles.
Then he left Aqua in the rain in front of a Cabin numbered three.
Aqua's memory had been wiped, but the only thing she remembered was being sent through time to here. That's the only thing, other than the image of her mother, who she thought was still alive.
Aqua was taken in by a boy named Percy, and later claimed as Poseidon's daughter, much to everyone's surprise. She stayed at camp for six years, but then finally went out to do some real saving.
I mean, when New York's being attacked by a certain mischief god, a d huge metal whales, why not invite a dragon into S.H.I.E.L.D?
(lol, i put her in the Avengers XD anyway, more of her story in the next pic. probably tomorrow.)