When you research the history of the question words in English, you find out something absolutely fascinating. It turns out, there was another question word related to <who> called <whon>.
Let me explain. First, you have to have a little background on where the <wh> that starts those question words comes from. It stems back to an ancient, unwritten word that linguists have reconstructed as <kwo>. This word was used 5,000 years ago on the Pontic Steppe by a large tribe of herders called the Yamnaya. (We call them that, they probably had their own name for themselves.)
The Yamnaya started sending bands of young warriors east from modern-day Ukraine into, and all over, Europe. The question word, <kwo>, over the next few centuries, changed sounds in some areas and gained spelling everywhere that written language was invented.
In the Latinate family, it gained the spelling <qu> and, once attached to various suffixes, went through Old French into Middle English and left us with words like quest, query, conquest, and question.
Meanwhile, in the Germanic family, the sound changed to a softer sound with an aspiration in the front of it, more like <hwo>. This word came to England with the invasion of the Anglos, Jutes, and Saxons. When the Normans invaded, the monks who wrote in English reversed the <hw> digraph to <wh>, so it would match the other digraphs in English that end in <h> (<ph>, <th>, <gh>, and <sh>). That left us with the word <whā>, which would have been pronounced /hwaa/, with a broad A sound.
At this point, it’s important to bring up that the word <who> is, among other things, a pronoun, and all other question words are forms of that pronoun (like <he> and it’s forms, <his> and <him>), or that pronoun attached to an affix (like the form of <him> with <-self> attached, <himself>). Only, instead of knowing what person, place, or thing you are referring to, the <wh> pronouns refer to an unknown object.
I’ll give you a minute to wrap your head around that one.
Now, just like the other pronouns, the first letter is a consonant sound that dictates the type of pronoun it is. An <h> means male, an <sh> means female, and a <th> means neither/either or plural. The rest of the word is a vowel that changes based on context you’re using it in. The <wh> in question words means, “unknown.”
There was a form of <who> that referred to a small number. That word was <whon>. It seems like it was replaced or just tossed aside over time, probably for the same reason that there are no gender-inflected cases of <who>. If you don’t know who you are talking about, how would you know how many they are, or which gender they are?
