My older brother is the only one of my siblings I'm going to compare myself to, though I have a younger sister and brother as well.
He is about 2.5 years older than me. He's in college and I'm a senior in high school. We are relatively similar physically. I'm a bit taller, with lighter hair and a heavier build. In terms of personality, we're a bit of a mixed bag, and I've always suspected that there might be some medical issue with my siblings in myself that produces oddballs, because it seriously cannot be normal with any of us.
I tend to be the one who always says "yes", so I have experience doing chores such as cooking, cleaning, etc. He is always a critic, and is unafraid to speak his mind (to a fault, actually), which means he's the one who argues with people and ends up doing less around the house. He has never, for as long as I have known him, found fault with his actions and apologized. Ever.
We have similar interests, we both are really deep into nerd culture, although I tend to get more gung-ho for things than him, and he really doesn't watch many American shows any more since he discovered anime. I tend to be more comfortable with displaying my personality. I'll fill my room with posters and such, buy nerdy clothing, etc., whereas he is pretty subdued about being a nerd, which is not to say he's ashamed, he just doesn't feel the need to display it.
We're both pretty smart people, although he has always gotten better grades than me (except for the AP English exam last year, where I scored higher and I still can't get over how great that feels). We're both pretty active from an extracurricular perspective, although I tend to stick with things and do a bit more. I am on the academic team at my high school and am planning a year-long trip to Spain the year after high school graduation in June, and he played soccer and was in the fencing club at his school last year for a while.
We both seem to go similar ways in life. He's independently studied Japanese to an astounding level in the last year (I actually think he might be in the neighborhood of having a workable vocabulary) and plans to move there when he leaves college. I'm studying Spanish, but I have plans to go after other languages (German, Arabic, Japanese) and spend my life abroad as well. We're both leaning towards engineering careers, although that has recently started to waver (towards teaching for both of us) on both fronts as we've gotten more interested in foreign cultures.
We're both socially introverted, having a small, close circle of friends that we really don't see outside of school very often.
He's a huge pain to deal with, but he's honestly the best friend I've ever had.