Who did it?
Who is that?
Who do you like?
I know who you are.
I saw a scattering of feathers on the ground, likely the scene of a predator catching a bird. I didn't ask "Who did this?" because I learned to generally use Who for people. What's a person? We both know the answer but it is still a valuable question. When you look into the eyes of one of the great apes, you can see a similarity to people you know. When you do that with a reconstruction of an ancient human such as a Neanderthal, you can see someone Who is a Who, although different from Who we are.
Some writing and thinking has been done on the emerging concept of an individual. I read a few years ago that knocking on the door of a rural Chinese farmhouse a couple of centuries ago when there was only an 18 year old servant inside would have brought an answer that there was nobody home. The writer conveyed the idea that the young woman actually did mean nobody was home because she didn't think of herself as anybody. She wasn't just humble, she was not a person. Today, we might add "she was not a person in her own mind" or "in her opinion" but it takes a few centuries to decide that such a matter is open to opinion and not a fact.
If you look up "the history of the concept of individual", you can find some of the emerging thinking about the subject. I think that roughly speaking the concept is about changing social ranks from truly and actually superior and inferior to more or less equal. It is not just a trumpet of liberty sounding, although emerging ideas of freedom and individual rights and abilities certainly are part of the story of Who. But, it is also the story of realizing that you are yourself and you like pineapples and can be a real pain but are also loveable. When I understand the facts and typical trajectories of you, I know you as a person, an individual unlike any other, before or since.
The Buddhists make a big deal of the fact that there is no Who in me, that when the scientists do an autopsy on me, they won't find the Self organ. Others are more likely to say that my habits, thoughts and body make a bundle conveniently thought of as Me but that Me is always being transformed by time, experience and conscious modifications such as study and practice. I can forget about Who I am or I can search for some more comfortable and capable version of Myself. Alfred North Whitehead and Buckminster Fuller were more likely to say I am a ongoing process.