Well, I figured out that I'm horrible at typing people IRL. So hats off to those of you with great observational skills.
I am unsure of the type of my cousin of whom I originally thought was ISTj, based purely off of an MBTI test and V.I. (the V.I. doesn't work anymore; the guy I thought he looked exactly like doesn't actually look like him very much.) I was discussing this with my INTj cousin, who's type I am confident of, and he said that the ISTj description doesn't fit him very well. After looking over the description again, I must agree that it doesn't fit him at all.
He is 15 and currently in 10th grade. His movements are quite loose and unaware. He more or less "falls" into each step, which gives him a characteristic waddle. His face is quite triangular, ending in a ball-chin point, and his eyes are a bit squinted. He was very, very quiet as a younger child and often spoke nasally. All throughout elementary school, he only had one friend other than myself and his brother.
Around me, he tends to be somewhat aggressive, where he will hit me in a victimizing fashion. And he does that because I never react to him. He is often the first one to quit an activity, getting bored with it and going off by himself to recharge. On that note, he often will, quite unexpectedly, want to be completely alone to do his own thing.
He is quite intelligent, and has a thirst for knowledge that matches my own. He is also quite curious, often asking me many questions about his topic of interest. He's a creative little fellow who often comes up with funny phrases or observations.
My relationship to him seems almost too perfect. From my standpoint, there is almost zero tension. We could hang out for hours and never really get bored, although we both have periodic bouts of wanting to get away and recharge. I could call the relation I have with him one of "mentor/mentee". He follows me in many things, often relying on my store of knowledge to build up his own. He also knows me almost too well. He can often predict what I will say in a given situation, even so far as to tell me what I was going to say (and he's almost always right.)
He usually doesn't take care of his appearance, and when eating, he can leave stains around his mouth, never realizing that they're there. Yet with other matters, he's not too messy nor too organized.
He matches me in my huge dislike soft or sentimental things. Whenever I show him one of my skills (like piano playing), he will often accuse me of thinking I'm "awesome" or something. He doesn't like when people become happy over something. He is also quite persistent at mastering a skill, but usually only in the initial stages.
Hope that was enough.
Your INTp friend,