Dexter and Daria are too non-responsive to people (i.e. directive or task focused) to be INTP. So I say INTJ for both and Mandark and Professor Utonium as well).
Double-D is an ISTJ Melancholy-Melancholy (Contender-Guardian or Chart the Course-Stabilizer) with an unusually high tertiary Fi [for that age. Hence, he's so much like an adult]. What appears to be a Fe social norms and ethics focus is really SiFi, but it just seems unusual for a young boy.
He is clearly more technically focused (a Thinker) on a level with the others I mentioned.
I've been further thinking bout the social norms issue, regarding Fe and Si. On one of my pages, I had awhile ago added:
Different aspects of behavior can be described by these different models. For instance, SJ is the temperament that values family cells, and such (Keirsey even associates it with a need to "belong to social units"!) This sounds like Fe, but it is not necessarly so; of course, because there are both SFJ's (who do prefer Fe) and STJ's (for whom it is in the deepest shadow). The common need among SJ's of both stripes is Si, for which a family cell (or other organization) meets their need for something familiar that matches their tangible storehouse of data, rather than an externally set set humane standard of judgment. So two different perspectives lead to a similar need.
I've been wanting to further add that the difference will be between a rational and an irrational method of reaching the same conclusion regarding a need for social order.
For the FJ, it will be based on actual
evaluations they rationally make, and for the SJ, it will be a matter of almost subconsciously just going with what they know, and they might not even really believe in it. This is especially true with STJ's (I grew up with them), with such things as family and holidays. As Melancholies (the ISTJ's, that is), they really can't be bothered with people much, and stuff like the commercialism of Christmas might actually go against their Fi. Yet for 'some reason', they plug on in the tradition anyway (Until they get older, and their Fi really asserts itself more). Here, you can really sense the difference between "rational" and "irrational".
Of course, SFJ's are both SJ and FJ, so they will be doubly into social norms.
Double D is not really merging his ego with the object (people, group) in humane evaluation. He's really splitting concrete data and extracting what's relevant to stored (subjective) knowledge, and also referencing stored humane values in evolving situations.
As for being 6, he does appear a bit Supine, and that could be a combination of being Supine in Affection (the area left out of the type correlation), and the first Melancholy (Inclusion) might actually be Melancholy-Phlegmatic, which would border on Supine, and thus make him border on ISFJ, and have both 6 and 5 tendencies.
Linus is the one who seem most likely INTP, perhaps Supine-Phlegmatic in Inclusion, and some sort of 5/6 combo. (He actually supports my notion of NT as Choleric, because he does have a great mount of stubbornness, as Lucy was calling him out on once (he tried to euphemize it as "tenacity" all the more proving her point; and another time, she even compared him to herself ("junior fussbudget" or something like that).
As for others, I'm not sure of any either. Batman does seem sort of like one though, and was a "hero" figure for me earlier on, with his logical deduction skills. Perhaps Robin, too.