Felix was an accomplice of the criminal, Vadim.
Felix helped rescue Vadim from the Chatelet. When he realised that D'Artagnan was joining their revolution, he didn't trust him at first. Vadim vouched for him though, and Felix was forced to believe that he was not a spy. Still finding him untrustworthy, Felix followed D'Artagnan out of the hideout. Inquiring about where he was going, he was told that he was going to see his mistress. Unsatisfied, Felix warned him that he would tell Vadim about the nighttime encounter, but D'Artagnan wasn't phased.
In a secret meeting of Vadim's followers, Felix was pleased to see D'Artagnan revealed as a traitor and punished for that treachery. At the Easter mass, Felix upheld his leader's wishes by throwing grenades towards the royal family. He was distraught to discover that they were duds. Enraged, he confronted Vadim personally. But, Vadim was able to stab in the heart first.
Felix has a stubborn way of discerning who is friends and who is enemies are. This leads to his naive trust in Vadim, who despite winning Felix's favor, betrayed him and later killed him. Felix is overall quick to judge and makes rash decisions, which eventually cost him his life. His foremost desires, however, are to fight for the people, and he has quite a free spirit, but that soon dissipates when he learns of Vadim's treachery.