Accomplice
noun
Someone who does crime together with you is your accomplice in crime. It's best to "take care of" these before the coppers get to him/her.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
あの共犯者は、まさに飛んで火にいる夏の虫だったな。
The accomplice was indeed like a bug that flew into the fire in summer.
共犯者は国外に飛んでしまった可能性が高い。
There is high probability that the accomplice has already fled the country.
彼はとても素晴らしい犯罪者で、自分一人でも、いとも簡単にとても危ない状態から抜け出すことが出来ました。そして、共犯者は一生必要ないとも豪語していました。
He was a really wonderful criminal who could easily get out of the most dangerous situations by himself. He boasted that he would never need an accomplice.