I still maintain Calvin is a introvert, though certainly not an extreme one:
1) The strips are held essentially in his inner world. It is essential to the comedy that his idea of what is going on and the rest of the worlds do not match up. Spaceman Spiff, the transmorgfier, turning into dinosaurs, Hobbes hiding under his desk to give him answers to test questions and doing his home work, all are apart of his inner world which no one else is privy too nor understands when they run into it.
2) Most all interactions with others are negative, interpreted through his inner world or both.
3) He is NEVER in a group, much less the center of attention of one, unless forced to be, such as in show and tell (where he basically puts on a act).
4) When he is noisey, he is usually alone, with hobbes in toruble or acting through one of his inner realites, repsonding to a inner-stimulus.
Thus I am certain the kid is a introvert.
Calvin's imaginary friends are his Intuitive function, not introverted.
The difference between Extraversion and Introversion is
not about how many friends you have, it's about energy expenditure (E) or energy conservation (I).
I'm starting to believe that a desire for friendships with real people is specifically related to ESxP or ExFJ types, since it's seeking enjoyment in the real world without a strong idealization.
An Extraverted Intuitive might not have as much interest in making real friends because their strong imaginations are able keep them company.
Calvin is clearly focused on experiencing the external world via Extraversion, but an idealized external world conceived in his Intuitive mindset.
One thing I noticed about ENxPs is that while they do talk a lot and are very energetic, they often just enjoy listening to themselves talk about their own conceptions and are often unaware of the concrete rules of socializing.
In their interactions with other people or things, ENxPs idealize their surroundings and are unhappy with the current way of things, so while they're Extraverted, their Intuitive function doesn't seek to enjoy, but rather to conceive. In fact, Extraverted Intuition's tendency to idealize everything, including people, could actually isolate the Ne type.
An ENFP will also seek deep connections with others due to their auxiliary Fi function, but since they are Ne types, those deep connections could possibly be with imaginary playmates (in Calvin's case, Hobbes).
I don't think that Calvin is an INFP. A dominant Fi child would probably be much more loyal and much less energetic. An INFP may have imaginary playmates because of their auxiliary Ne, but interactions would probably be at a much quieter level, maybe not interacting with them through acting out, but through writing.
And now I had a new thought. I think an INFP might actually be better at friendships than the ENFP. Their Fi is developed first, and so will seek a deep connection with someone else at an early age, they have the desire to bond with someone first, but when Ne is developed, they might perhaps engage in creative storytelling together.
But the key is that Extraversion isn't about friendships per se, but about direction. One can be extremely gregarious and exuberant in the world, but not necessarily with the intention of making friends.