Relative pronouns (who, whose, which, that, as) not only point back to a noun or a pronoun mentioned before but also have conjunctive power. They introduce attributive clauses. The word they refer to is called their antecedent. It may be a noun or a pronoun.

Who is used in reference to human beings or animals.

Jolyon bit his lips; he who had always hated rows almost welcomed the thought of one now. (Galsworthy)

…in his voice was a strange note of fear that frightened the animal, who had never known the man speak in such way before. (London)

Whose is mainly used in reference to human beings or animals but it may be applied to things.

Then there was the proud Rychie Korbes, whose father, Mynheer van Korbes, was one of the leading men of Amsterdam. (Dodge)

Again he (Soames) looked at her (Irene), huddled like a bird that is shot and dying, whose poor breast you see panting as the air is taken from it, whose poor eyes look at you who have shot it, with a slow, soft, unseeing look… (Galsworthy)

…he (superintendent) wore a stiff standing-collar whose upper edge almost reached his ears, and whose sharp points curved forward abreast the corners of his mouth… (Twain)

Which is used in reference to things and animals.

Here was her own style — a bed which did not look like one and many mirrors. (Galsworthy)

They strove to steal a dog — the fattest, which was very thin — but I shoved my pistol in their faces and told them begone. (London)

That is mainly used in reference to animals and things. It may also be used in reference to human beings.

This… gave him much the same feeling a man has when a dog that he owns wriggles and looks at him. (Galsworthy)

On one side was a low wall that separated it from the street. (London)

In the factory quarter, doors were opening everywhere, and he was soon one of a multitude that pressed onward through the dark. (London)

As usually introduces attributive clauses when the demonstrative pronoun such is used in the principal clause (it is a rare case when as is used without such in the principal clause).

As may refer to living beings and things.

…perhaps the books were right and there were many such as she (Ruth) in the upper walks of life. (London)

His mother was a poor peasant woman, too poor even to think of such a thing as buying skates for her little ones. (Dodge)

For nobody’s ever heard me say as it wasn’t lucky for my children to have aunts and uncles as can live independent. (Eliot)

…I went into Snow Park. It wasn’t as one expects a municipal park to be…(Braine)

Relative pronouns can also refer to a clause (see Complex Sentence).

Relative pronouns always perform some syntactical function in the clause they introduce.

Gemma, there’s a man downstairs who wants to see you. (Vovnich) (SUBJECT)

She flashed a look at him that was more anger than appeal. (London) (SUBJECT)

…then discussion assumed that random volubility which softens a decision already forced on one. (Galsworthy) (SUBJECT)

I think I have taken nothing that you or your people have given me.(Galsworthy) (OBJECT)

Families often think it due to themselves to turn their back on newcomers, whom they may not think quite enough for them. (Shaw) (OBJECT)

It pleased Denny to exert the full force of his irony upon the work which they were doing. (Cronin) (OBJECT)