Calvin can't possibly be I in general, due to the need to express himself externally. Even though hobbes is an aspect of his own mind, as is most of his imagination scenes, the fact is that he expresses himself externally rather than internally in thought process.
Therefore, I is pretty much impossible.
He's definately E to begin with, most likely N due to understanding things as complex patterns intuitively rather than seeing whot's really there (N over S), his choices and thoughts are generally described through emotions, but he also puts alot of thought into emotional matters (albeit after translating them heavily through Ne into caricatures), so I'm 50/50 on whether he's F or T, since he shows tenancies of both when making decisions or thinking of things. The P is almost guaranteed; he has no desire for finality, nor order. Calvinball itself, his own game, has only one rule, that it can't be played the same way twice. That pretty much kills any belief in J.
So most likely, he's either an undeveloped ENTP, or an ENFP. The F seems more likely, but considering we're talking about an undeveloped child, ENTP's at that age tend to show more F signs early on, so it's hard to be certain.
Either way, Ne is his primary function definitely, regardless.
This brings us to the matter... Ti or Fi?
Fi filters information based on interpretations of worth, forming judgments according to criteria that are often intangible. Fi constantly balances an internal set of values such as harmony and authenticity. Attuned to subtle distinctions, Fi innately senses what is true and what is false in a situation.
Ti seeks precision, such as the exact word to express an idea. It notices the minute distinctions that define the essence of things, then analyzes and classifies them. Ti examines all sides of an issue, looking to solve problems while minimizing effort and risk. It uses models to root out logical inconsistency.
Well he lives in such an imaginative world, that his grasp of 'true and false' is kind of lacking most of the time. I would argue this to mean it's unlikely he maintains much Fi.
Since he interprets things into the 'essence' of them, and breaks them into metaphors (his imagination) to better express them and understand such, as well as goes to great lengths to minimize effort (aliens ate my homework! Gravity reversed, I couldn't do anything!), and his models are all based off Ne, which means the logic he uses is consistent within his own imaginary world, I would suggest Ti is more likely than Fi as his secondary function.
This would imply he's probably just an undeveloped ENTP being mistaken for a mature ENFP.
