Gilles de Rais/Lavalley/Martinet is one of the most intelligent/ruthless antagonists in season one. This sorcerer/demon was able to use the gods' negative attitude (including Gabriel's) towards Jeanne in order to manipulate Jeanne to join his cause and open the seal of Bahamut. A lot of fans will say that Martinet is an antagonist because he is the one to open Bahamut's seal, which is true, but people don't understand what events led to his plan succeeding. For instance, a lot of fans will argue that Martinet took advantage of her, but remember Martinet did mention that the gods did betray Jeanne. And he isn't wrong because like I said about Gabriel and the gods, they never gave a "chance" to become the Holy Knight, nor was she considered the gods' equal. In reality, she was nothing more than a "mere instrument" and pawn to further the gods' agenda. That is harsh betrayal, isn't it? Even Martinet notices Jeanne's closed-minded thinking about her becoming the Holy Knight and asks "Is that how the gods tricked you into this? Why you're nothing but sort." Jeanne gets scared in prison because even she is beginning to realize that her relatinoship with the gods wasn't always that positive. I would like to add that in episode 3, when her second-in-command Lavalley asks Jeanne whether they were were protected by the gods or not, she glares, but confirms Lavalley's fears with the statement, "I fear that we're not." (Remember when Lavalley/Martinet/Gilles De Rais heard those words, that is probably how he managed to turn her words against her). He understands what Gabriel and the Gods are capable of, especially when it comes to putting humans beneath them and controlling them as pawns. And that is why he was able to use the gods' attitude towards Jeanne as an advantage as part of the plan to opening Bahamut's seal. Let me remind you that when the angel Michael died at Jeanne's hand, he understood that it was their arrogance that led to Bahamut's revival. That is how all of this happened. 

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