(in the meantime, check fenshen@tumblr for post-debut updates, or his part of the exo pimp post over at exochocolate)
Sources are pretty much all from his baike page, his forum (?), and his baidu bar.
So Chiharu has been asking me why I love 张艺兴 (also known as LAY, thanks SM) so much. I was only half joking when I said it was ~love at first sight.
BUT. There are so many reasons why he should be loved!
As far as SM goes and EXO goes, Yixing is still a relatively new face, but as far as Yixing goes, he's not only not a newbie, but has quite a history already. He's danced with SHINee, he's danced Nobody, he's been on TV, and apparently was somewhat of a celebrity at his school.
Some quick stats (from his baike page aka accuracy unsure):
Name: 张艺兴 / Zhang Yixing / LAY
Birthplace: Hunan Changsha
Blood Type: A
Birthday: 1991, October 7
Height: 179 cm
(He also likes playing on the computer, likes all movies, all colours, all good tasting foods, and playing basketball. His most unforgettable moment was being on a TV show for the first time, his favourite celebrities are Super Junior, DBSK, SHINee, and JJ Lin, and his biggest wish is to open a 指纹店 for his mother.
☄ SHINee World First Concert
I think this is where I first noticed him and then looked him up and then...forgot about him ;;
WAY BACK WHEN, SHINee had a concert and Jonghyun couldn't dance. And while I'm sad Jonghyun couldn't dance, now I'm a little thankful too because that means that Yixing joined SHINee on stage a lot. And I mean a lot. I could embed every single one of them but I'll just leave links at the bottom :) In the meantime, have him dancing Senorita
SHINee World First Concert - Senorita
☄ Nobody (+ other)
He has a step-up in the idol world—he's already danced to Nobody and he looks so blissed out doing it.
Nobody + Other Dancing, 2009
HE NEEDS TO DO THIS ON A SHOW OKAY.
Prior to SM, this was what he was most well known for. 明星学院 is this sort of...idol competition thing I DON'T KNOW ONE OF THOSE TV PROGRAMS and okay all I knew about it before 张艺兴 is that they interviewed 韩庚 for it at one point. BASICALLY there's a bunch of kids and they do stuff and uh he ended up coming out of the 2005 one in third place. Here is his profile from that. LOOK AT HIS CHEEKS. LOOK AT HIS ROUND FACE.
This is a clip of him performing Cyndi Wang's 爱你 on the show:
He's also apparently somewhere in this video which is the theme song for the show but I am bad at spotting people please help me find him ):
In 2006 (I'm guessing) he was invited onto 那可不一样 having previously won 3rd place in 2005 along with the other 2005/2006 winners. Here is a tudou link to the first half of that episode. He comes on at ~1:58 and DO WATCH FOR THE REACTION. His actual part is at 22:51 and onwards. BASICALLY GIRLS THROW THEMSELVES AT HIM AT SCHOOL (okay not literally but you know). Other tidbits of information, he sort of dieted between 2005 and 2006, ~focused on school in the meantime and didn't work on his entertainment side, and apparently when he went to his school for a concert his fans were crazy. Yup. Boy was famous already. Also his voice breaks when he sings and it's great. (Eng subbed version embedded below!)
He's written a number of songs, with two linked on his baidu page. The source for the second one is this and it has a dead link, I'm just going to talk about the first one for now:
想说心里的话给你听 - 张艺兴
Basically he is young and cute here and I forget which show it was from but I'll add that info when I dig it up again, and I'm pretty sure he's a better singer now than he was back then. Lyrics and quick translation are here.
泪水机场 - 张艺兴 (2010)
my girl - 张艺兴 (2010)
想你想你 - 张艺兴 (2010)
☄ Yixing's Words
Thing's he's written, messages to fans, on the bar, etc.
张艺兴's messages on his bar (2007 - 2011 + translations)
A series of posts he wrote back in 2004
Which may be expanded into real words sometime but for now HAVE LINKS:
XingPark's Youtube channel!!! (his forum/baidu bar)
Newspaper article on him 
Pulling votes on 明星学院
Playing piano on 明星学院
Clip on SM Trainee (he is in here somewhere and I'll find YT embed later)
'fancam' of another dance perf at same venue as Nobody perf
Him and bros WALKING ACROSS THE STREET (yeah that's right it's worthy of a video link)
SHINee World - Lucifer
SHINee World - Ring Ding Dong
SHINee World - Juliette
(and he has been on other shows, became a sm trainee in 2008, and stuff. i need to go and dig up links again -.-)
(temporary link dump. hopefully temporary:
He is awesome and you should love him with me! But beyond that, he clearly has a strong variety sense, experience, and a variety of talents. I'm really interested in seeing if SM will push his songwriting as part of EXO-M (...I'm guessing here) and I'm incredibly excited to see him back on some shows in China. And if it's a show in Korea...I'm sure he can pull out Nobody, and all will be well. Oh, and he's a dancer. And he has nice hands.
thank you for the light you have given me