Quadrophenia : Who is the girl…

that Jimmy sees when he is sitting on his bike by the canal? He shines his light onto her then rides off angrily

If you're talking about the girl and guy he sees kissing (shines his light then rides off) he was simply mad because the girl he fancied took off with someone else from the party.

He didn't want to see a "happy couple" anywhere near him (even though he didn't know them) and decided to be a jerk.

http://werewolvesbeatingadeadhorse.blogspot.com/