In most first instances, you would tend to get into the sarcasm mood. In such a mood, you would call that person an accomplice.
Accomplice relates to a partner who is totally with you into doing everything that is wrong, usually used as a legal term in crime cases.
However, accomplice also usually used when a couple "goes postal" on themselves, without a care in the world doing whatever they like, breaking any rules if necessary.
Mark and I will be in Las Vegas for Thanksgiving week. He has a lot of unplanned ideas up his sleeves, and I will be his accomplice.
The word accomplice then brings us to the word abet and the abettor. Even though abetting is usually used in criminal cases, it actually mean cooperating and encouraging actions whether good or bad.
Which then brings us to the word accessory. An accessory is someone who goes along with the perpetrator's ideas. She/he does not have the originality or motivation to devise or cook up ideas. She/he simply agrees to go along with it, giving every encouragement, cheering and whatever cooperation necessary to the perpetrator.
Satyanatan is a brilliant architect. He has had such vast experience, such depth of creativity. You see all these works we "designed together", all the awards we have won? I have merely been an accessory to his ingenuity and went along with all his brilliant ideas.
Dictionary lookup:
accessory (əkˈsɛsərɪ)
n, pl -ries
- a supplementary part or object, as of a car, appliance, etc
- (often plural) a small accompanying item of dress, esp of women's dress
- (Law) a person who incites someone to commit a crime or assists the perpetrator of a crime, either before or during its commission
adj
- supplementary; additional; subordinate
- (Law) assisting in or having knowledge of an act, esp a crime
[from Late Latin accessōrius: see access]
accessorial adj acˈcessorily adv acˈcessoriness n
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003
accomplice (əˈkɒmplɪs; əˈkʌm-)
n
- a person who helps another in committing a crime
[from a complice, interpreted as one word. See complice]
Collins English Dictionary – Complete and Unabridged © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003
a·bet (-bt)
tr.v. a·bet·ted, a·bet·ting, a·bets
- To approve, encourage, and support (an action or a plan of action); urge and help on.
- To urge, encourage, or help (a person): abetted the thief in robbing the bank.
[Middle English abetten, from Old French abeter, to entice : a-, to (from Latin ad-; see ad-) + beter, to bait; see bheid- in Indo-European roots.]
a·betment n.
a·bettor, a·better n.
The American Heritage® Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.