Gadfly

noun

1. Any dipterous insect of the family Oestridae, commonly known as botflies.
2. A horsefly: any of various species of fly, of the family Tabanidae, noted for buzzing about animals and sucking their blood.
3. One who upsets the status quo by posing upsetting or novel questions, or attempts to stimulate innovation by proving an irritant.
4. One who merely irritates without making useful suggestions.
5. A bloodsucker; a person who takes without giving back.

   "He's a regular gadfly and takes advantage of his friend's generosity."