If you keep in mind that WHO always refers to people the real problem becomes when to use WHICH versus THAT.
There are lots of explanations online about choosing the correct pronoun, many of them referring to restrictive or nonrestrictive clauses, etc. To strip it down to the bare essentials remember this: THAT refers to one thing among many.
In this way it is similar to the indefinite article A (AN).WHICH refers to one specific thing.
In this way it is similar to the definite article THE.
I would like to buy a dog that comes when I whistle. (I want a dog with this quality, but I haven't found it yet.)Note
I would like to buy the dog, which comes when I whistle. (The specific dog I want to buy demonstrates this quality.)
: A clause using WHICH can often be left out of the sentence without compromising the meaning.
Her bike that she painted yellow is for sale. (There are many bikes and it is the yellow one that is for sale. One among many.)
Her bike, which she painted yellow, is for sale. (There is only one bike for sale. It happens to have been painted yellow. Notice the commas to hold the nice to know information in place with the need to know.)
Since there is only one bike for sale, I may not need to mention the colour as I do not have to distinguish it from the other bikes.
The clause beginning with WHICH could then be removed: Her bike
, which she painted yellow, is for sale. Or, Her bike is for sale.
Hope that helps.