stooge

Short definition from Wiktionary

Noun

1. 1
   One who knowingly allows himself or herself to be used for another's profit; a dupe.
2. 2
   A magician's assistant who pretends to be a member of the audience.
3. 3
   A straight man.
4. 4
   A secret informant for police.
5. 5
   A confederate; a person who is secretly working for the researcher, unknown to the study participant.

Verb

1. 1
   To act as a straight man.