Disclaimer: I don't own the characters of BTVS or Sanctuary. They are the property of Joss and Syfy.
Approximately twenty years ago Nikola Tesla, the vampire genius, created an experiment. He wanted to create himself a vampire son that would be superior to humans. For that he used his own vampiric DNA and DNA he got from his long living friend Helen Magnus. Because Helen would never willingly participate in such experiment, he had to steal her genetic material. If his experiment had succeeded, he wanted to create a new vampire race that would take over the world. However, the experiment failed. The child that should have been a vampire was completely human or so he thought. Nikola abandoned the experiment and after some thoughts he abandoned the child as well. He didn’t really wanted to leave him, but with his life and his work, he couldn’t raise a completely human child. His work endangered him a lot and a human child wouldn’t be safe with him. So he left him in a public place where he knew that someone would find him.
The child was later adopted by a normal human family. His was named Alexander LaVelle “Xander” Harris and he grew up thinking he was a normal human. And he was, to some point. His childhood was spent in Sunnydale on the Hellmouth. The couple that adopted him, regretted the adoption after a few years, but knowing they couldn’t do anything about it, they still raised him thinking he was their son. When he was younger, he was often terrorized by his adopted father Tony, who started drinking when Xander was very young. His adopted mother didn’t defend him and so he was often hit by Tony. He had two best friends Willow and Jesse. He met Willow in kindergarten and when they were in primary school he often took refuge in her house, knowing that her parent weren’t home a lot.
Until he was sixteen he didn’t know anything about supernatural beings. He thought that Sunnydale was just a place with a lot of gangs, often high on PCP, and a lot of people, who didn’t really know how to cook and were really clumsy, which often led to barbecue fork accidents. But then Buffy came to town and he and Willow found out about other creatures that roamed the Earth. Buffy was the Vampire slayer and Giles, the school librarian, was her watcher. After Jesse was turned into a vampire and staked by Xander’s hand, the two friends started helping Buffy and Giles. Everything was normal or normal for them, until the summer after graduation, when everything changed.