A leacher is an unimposing person that always accompanies another person which in most cases is very beautiful and self-confident. Leachers do not have a social situation for themselves, instead they are taking benefit of the person they are conducting. Most beautiful girls are surrounded by at least one leaching girl. Strong men also have leachers, mostly wimps trying to take benefit.
"Hey PJ, lets get out to have some fun." -> "But who is gonna drive?" -> "Hmm, lets ask Lauch, he sucks but he has a car."
In this example everybody is happy, two guys getting a car plus a chauffeur and one guy (the leacher) getting social life.