It’s been a while since I’ve done one of these. So here we go!
I is related to the Greek letter iota (Ι ι). As you can see, the only thing missing is the dot above the lower case. If you look at the Alphabet gif, you can see its appearance changed a lot over the centuries. Before being a straight line, it was a lightning bolt like shape and before that, it was a shape quite similar to early sigma.
The Greeks adopted their alphabet from the Phoenicians. Although they didn’t indicate vowels, yodh, I’s origin letter was a part of their script. Yes, it was a consonant. Yodh also has a hand in creating Y and especially J (that’s the reason I and J look so similar). I assume it didn’t become a vowel until the Greeks started up their own alphabet and designated it so.
Finally, in the last bit of I’s history that I could find, we can see the reason for yodh’s shape. The Phoenician alphabet comes from Proto-Sinaitic. Like all Sinaitic letters, it’s based on an Egyptian hieroglyph. In this case, the one for arm.
There is one more bit that is related to the letter: it’s use as a pronoun. In Proto-Germanic it was ekan, which came from the Proto-Indo-European eg. Even in English it was ich up until the twelfth century. And the being capitalized thing? To keep it distinct in flowing script. That little i is very small after all.