Partly is a light blue pegasus with a soft, fluffy white mane (which may or may not smell like pudding from his sister's latest prank). His cutie mark is a rainbow behind three clouds.
You barely ever see Partly without either: A) His sister, B) A good book, C) Something other than another pony nearby.
Partly is the shyest (SP?) pony you'll ever meet. Partly has always been hiding behind the cover of his sister, Chance, ever since he can remember.
Partly, much like his sister, is extremely nice. It's very hard to get him angry or to get him to retaliate, so watch out. You never want to lose Partly's friendship, he's the best pony you'll ever meet. He's also the greatest pony at keeping secrets, so watch out. He might seem very innocent and know-nothing-ish, but he knows a ton.
Partly, unlike his sister's view of their history, had a terrible childhood. In Hoofton Kindergarten, he was thrust into the Earth Ponies class because of his flight skills; they were absolutely terrible. And usually, since he was a different race than the earth ponies, he was picked on and bullied. Partly would always tell his parents but they'd never do anything for him. Chance would help (mainly by pranking the bullies, which got her a cutie mark) but it was never enough.
Years passed, Partly didn't get his cutie mark. He went to Hoofvard college and got a doctorate in Psychology (even though his player has no idea what that is).
After a while, he quit on getting his cutie mark, thinking he'll be one of those 'blank flank forever' kind of ponies.
Soon after he graduated, he found out he was gay. It was tough to swallow, being that he was too scared to tell anypony, but he hid it for a while.
One day while he was working with clouds (being that he got progressively better at flying) he sat down and stared at the sky. A rainbow had formed (a spill of rainbow juice) and he just sat and stared. He didn't know that somewhere behind him his parent's house was crashing down upon his parents.
He was still sitting there when he got the news of his parent's passing and didn't move for hours, just staring at the ponies cleaning up the rainbow. He realized that they were fixing their mistake and would probably hide it. Somehow, thinking about that, Partly realized that he never told his parents he was gay.
For this reason, Partly got his cutie mark, a symbol recognizing that Partly hides behind anything and everything (his sister, a book, his parents sometimes, and when he wants to be alone: some clouds).
To this day, Partly has still not told his sister that he is gay.
Now Partly lives in Ponyville with his sister Chance. He still hides behind her, doesn't go out much, and is terrified of earth ponies.
Mom: Gleaming Cloudy (Deceased)
Dad: Hurricane Cloudy (Deceased)
Sister: (Cloudy With'a) Chanc'a Meatballs