From Headcanon Meme (w/ additions)

ALL crossed out items have been jossed by canon.

UPDATED: June 29, 2014

1. The name on his birth certificate reads Llywelyn Krzesimir* Sasha Stilinski. (Stiles is from his last name, obviously.)

*To spark/light/rouse + peace

2. He has Welsh heritage from his mom's side of the family. Polish. He grew up hearing stories about the likes of Ivan, Czernoborg, Baba Yaga, and Koschei. As well as other folkloric stories and common fairy tales.

3. He and Scott met in grade two - they were supposed to be spelling words with wood blocks, building castles was better. Worth the trouble they got into for hogging all the blocks.
( There may be spoilers )