This is my cousin. We live across the street, and there were no young kinds in our neighborhood, so we used to hang out, even though he's 5 years older than me. I'm thinking ISTP, although I'm conflicted about his emotional spits: they may be from drugs or hangovers, but they may be his personality instead. In which case, I'm not sure... Nonetheless, I know an ISTP (over the internet though), who is completely different than him. But anyway, let's read the below column of information before getting too deep.
I'd assume that he hadn't had any friends until 4-6th grade or so, although I can't be sure about that. I know that he used to glue and paint various models, play with Lego (mostly war stuff), play with soldiers, wasn't much attracted to Monopoly. We used to play Monopoly, cards and Lego together (seldom though, he preferred to be alone). Later on, we were watching TV together, he used to like Sci-Fi movies, like Star Wars; he also read Sci-Fi books. His grades weren't the best, even though he was had extra teachers at home for a few subjects. Later on, when I was in about 2nd or 3rd grade, we still used to hang out, but he was turning a bit "violent," I often turned out to be crying under the bed, as I was little. Perhaps he didn't mean to do the "bad," although I believe sometimes he didn't understood what's the deal with me, why I couldn't take a hit or something like that. Later on, up until probably 6th or 7th grade, I went to his place and watch him play games, he didn't mind me, sometimes even invited me. In the very end, he started expressing his emotions towards games, like hitting the table and stuff like that. He didn't let me play even if I'd ask.
He smokes at least since he's 18, though I bet he started smoking when he was 16 or so, same goes with drinking alcohol. I believe he preferred a few friends over parties, even during the graduation party he went to get drunk with a few friends instead. I believe that since around 18 or 20 he uses some kind of drugs, ones that relaxes him, and ones that energizes him; I'm almost certain it isn't pot. He also started going to clubs or bars, although I'm not sure how often, I've just seen the stamps on his hands a couple of times (I meet him rarely though).
Since he was about 20-21, he increasingly asks about mine and some other peoples' lives, trying to advice what to do, trying to play the family role. However I'm not sure if he's always on drugs, has an extremely poor memory, or doesn't really care and is pretending. Or perhaps he's trying to score with his father, who's also interested in our family matters (he's my mom's brother though, so it's more "natural"). Whichever is the case, he increasingly used to ask how's my grades, how's school, if I'm alright socially, if I need someone beat up and stuff like that. He seems to pass that info to his father, I'm not sure if he passes everything, but most of what I tell him goes to his father's ear.
At the end of the 12th grade, and maybe 11th grade, he studied a bit harder and instead of 5s/6s/7s, he started getting 8s/9s, and a couple 7s or so of course. Then he went a college and after that, he got his master's degree. He seems to hate school, but he thinks that "you have to finish school good to have a good life," job-wise, I believe. He didn't go much to school though (earlier), I remember him constantly asking his mom to sign that he didn't go to school for a reason and not just skipped classes, even though he did go with his friends somewhere instead.
He seems to believe that he is right until proven otherwise. Even if I tell him an exact reference in a book or a site, or what a teacher has told me, or even what his dad told me, he doesn't believe me until he looks it up and sees for himself, although usually he doesn't bother and just keeps to his beliefs.
He doesn't seem too smart, as I often have to explain what I mean in more detail. But so I have to do to most people, just a few people understand (and understood) me since I first spoke to them.
He seems to constantly offer help in social situations (like the "do you need someone beat up" part), or something along the lines of "does anybody laugh at you at school." Just a couple examples. However, when recently I borrowed some money to a friend who became a pothead and refused to give me back my money, he refused to do something about it. He was talking about it for half an hour twice, going how it can't be solved peacefully and he doesn't want me to get hurt, but I believe I could hear in his voice that he just didn't want to get involved and do anything.
He chose to be an accountant, but that's his father's profession. I've heard him say that he hates his job. I asked him recently what he's gonna do about his place in life, he said that he's gonna continue his education, get some papers that he can speak English (as he took Russian instead at school), and later he said that he's "got it all planned out." I heard confidence in his voice, although I believe that he isn't sure, instead he only thinks that he's sure.
He seems to cling onto old things (things that he knows), for example old behaviors of mine or somebody else's, even though that was 10 years ago; or he's always repeating that "history is important, why didn't you take the exam" to me, even though the standards had changed over the years.