Miroku is the second person to join Kagome and Inuyasha on their quest to find the Shikon no Tama shards. He caused a lot of trouble for everyone, especially since this was the first time that they had met a non-youkai individual who was looking for the shards for a purpose that wasn't evil. Miroku kept to himself for a while but after the incident with the artist who was making many oni out of ink, Miroku became a permanent member of the team.
Miroku, to put it lightly, can be summed up as a letch. He may be a Buddhist priest but he certainly doesn't act like one. He uses his status as a priest to manipulate rich families into giving him free food and board and on top of that, he's an extreme womanizer. Miroku takes every chance he gets to ask any beautiful woman he comes across to bear his child. Unlike other famous letches in anime (ie: Ataru from Urusei Yatsura, Carrot from Bakuretsu Hunters), Miroku's letchery often goes rewarded to some extent, much to the displeasure of Sango.
Despite his delinquent antics, Miroku is a valuble asset to the team. His skills and knowledge alike help out in more ways than one and without him... well, the series just wouldn't be the same, would it? ^_-