Mark "Rents" Renton: INxP
Sick Boy: ENTx
Begbie: ESTJ
Spud: INFP

Irvine Welsh, author. Admits that Renton is basically him and that Renton represents the naturally selfish young man made even more selfish by an addiction. States that empathising with the characters (even sociopathic ones) is vital to his work.

Leaning INFP. Same for Renton. A twisted INFP perhaps.