I think of all the Buffyverse characters, once stand out as being the most "normal". He is the one that, I think, most of us could identify with, especially in our high school years. I think for that reason, Xander Harris is probably the most human character of them all.
I think what set Xander apart from the rest of the Scooby Gang, especially in later seasons, was that all though he was part of the gang, he was always a little outside of them as well. Despite his military knowledge, borrowed from his Halloween Costume, Xander didn't have any special powers. He wasn't super strong like Buffy, didn't have Willow's affinity for magic, and he definitely didn't have the brains that Giles was gifted with. What he did have though was a fierce sense of loyalty and courage that I'm not sure I would have been able to display if I was in his shoes.
It was that lack of "specialness", at least in his own head, that made Xander's struggle with maturity that much harder. The entire arc of his character was him trying to grown up, become a mature, adult male who could not only take care of himself but others as well. He started off as a little geeky, but like a lot of socially awkward people, he had all the best lines. He had a sense of wit that I, as witty as I am, was just a little bit jealous of. His lack of maturity though, and lack of a filter, would allow his mouth to get him in trouble sometimes, but he was always funny.
As the series progressed, especially with episodes like "The Zeppo", Xander started to become more confident in himself and his role in the gang. He took a more ownership of the decisions and by the last season, he was an almost unofficial Watcher for Buffy. Like most of us who had that awkward high school stage, Xander eventually discovered who he was and became an adult who was a productive member of a group that acted more as a family than anything else.