Our beloved Nicky was born alongside his identical twin brother Kelly, on April 12, 1971 in Los Angeles. (where they grew up) Kelly was three minutes older than Nick, and that always translated into frowns at birthday parties. Kelly would get his cake first, and by the time they got around to Nick's, nobody was interested. (cause they had Kelly's first) Nick was the luckier one in grade school though. Their mother color-coded their wardrobe, so Nicky was dressed in blue while Kelly was stuck in brown & yellow.
By the time they got to high school, Kelly was popular and did the party/girlfriend thing, while Nicky did not. He was girlfriendless all four years, had acne, and braces. On top of all that, he had a stutter; caused by his insecurity. One thing Nicky did do was play baseball. His junior year (1988) he was the starting player on the nation's top-ranked high school team. He had his heart set on playing for the Los Angeles Dodgers, but a few years after graduation, he injured his arm and decided to try acting. 
Nick landed a few bit parts in commercials such as Clearasil, Burger King, Sprite; but he grew frusterated and tried the college thing. He even considered medical school. But, when he was twenty-five, he dropped out of school and landed the role of Xander on Buffy the Vampire Slayer. (for once halting your education is a good thing!)
Now, Nick and his brother live together in an apartment that is very nicely done. (as seen in pics!) When describing their relationship, Nick says that he always can count on Kelly. For instance, on Nick's first day of Buffy, he didn't know when he had to shoot his first scene, so when the time got there; he had to shoot a heavy dialouge scene with Sarah and didn't know his lines. But, he called Kelly, who calmed him down and gave Nicky a fresh approach to the day. And, as we can see, everything turned out fine!
Nick can also always go to Kelly with girl problems. When Nick was broken hearted leaving a very intense relationship; Kelly took him out for a drive and just let him cry. Yes, he cried. Nicky seems to be very sensitive.
Nicky has a sensitive and slightly sadistic side. It is known that, sure, he cried when he broke up with his girlfriend, but here are some rougher more taboo tidbits: He smokes. He drinks. He loves dirty jokes. (read any chat transcript and you'll see) He loves to go to strip clubs with David Boreanaz and Anthony Stewart Head. (only once, as far as anyone knows!) 
As for relationships, I only know of one. His most recent long term relationship was with Wendy Thompson. It apparently lasted for more than a year, but his most recent statements infer that they broke up. So, he's single! (***note***: I am not blatantly happy that he is romantic-personless, I want him to be happy. But, any delusional person, such as myself (kinda) can't help but be slightly thrilled that he's not with anyone.)