Why is he called Littlefinger?
Lord Peter Belish is called Littlefinger by the brother of Lady Catelyn (Ned's wife).
It is said that there were two reasons for that.
Lord Belish's father only had a very limited area of control in the Fingers. The other thing is that he inherited a body smaller than his age.
Lord Belish, or Littlefinger, grew up in Riverrun. He has been looking forward to marrying Lady Catlan. Ned Stark's brother, Brandon Stark, tries to kill the man, but Catelyn is the one who solves the problem.