In a sentence, when would 'who' be classed as an adjective?
'Who' does form the headword of an adjectival clause, which then qualifies a word or words in the main clause. Making use of 'who' as part of an adjectival clause is adding additional information. The following example should help you:
The girl who wore the red dress looked the prettiest.
The adjectival clause is the bold section and starts with 'who'. The main clause is 'The girl.....looked the prettiest.' So you can see that the section beginning with 'who' adds information and actually describes (the role of an adjective) the girl.
'Who' most often, though, would be classed as a pronoun and more imporantly, an interrogative pronoun - it usually asks the questions. It does get more complicated though as 'who' is also a relative pronoun:
This is my sister who is visiting today.
In the above sentence the 'who' is referring to the subject - the sister.