Taking in mind the definition of pronoun (in the previous chapter of personal pronouns) there are some other words too that may replace nouns, like; who, that, what, which.
Now if you look at the following sentences you will see that who, unlike other relative pronouns, can be used in three different ways.
Nominative use of who (where who acts as subject)
Accusative use of who (where who acts as object)
Possessive use of who (where who shows possession)
Relative pronoun [which] also has three uses but it remains the same in all of them. The difference between who and which is similar as is the difference between he and it. It is used generally for inanimate thing, animals or anything provided we are not referring to its gender. Which too is used for the inanimate things or animals. While who and he refer to person.
There is some interlocking of classes (parts of speech) with respect to functions that even without any derivation one class may correspond to different parts of speech, e.g. who is used both as conjunction and pronoun and it is owing to the same it is called as conjunctive pronoun.
Compound relative pronouns
Relative pronouns are sometimes used as compound of different words. Following examples will be helpful to understand this. Italicized and underlined words are compound relative pronouns.
Other forms of whatever is whatsoever similarly whoever and whosoever.
Who, whom, what, which, whose etc are also called interrogative pronouns because they are used in interrogative sentences also, like;