I saved Count Saint-Germain for second to last because I heard his storyline was very good, so I figured this would pick me up after Impey's route and carry me well into Lupin's.
Though it took me a while to get into it, it ultimately made a good impression. I was still thinking about the final chapters of his route days after I finished it. It wasn't perfect, but the ending made up for a lot.
The problem is that Saint-Germain is a difficult character to get a handle on. He's the mysterious man with an enigmatic smile, so you never really know what he's thinking. When I went through the shared route a fourth time to catch all the Saint-Germain specific scenes, I didn't get much to illuminate his character that I didn't already know. I didn't feel closer to him. And then there was the confrontation with Finis.
At the end of the common route, the love interest who Cardia is closest to, will interrupt the completion of Finis's plan and prepare to fight him off, only for Saint-Germain to suddenly show up and stab Finis from behind, killing him and ending the danger. This is the only time on most routes that Saint-Germain is ever depicted with his eyes open. And he's the only one of the love interests to actually kill another person in front of Cardia. (Van Helsing's got a lot of blood on his hands, but all of it's in the past.)
On all the routes, Cardia is shocked by the sudden death of Finis, who in a sense was her brother, being an artificial creation of her father's, just like herself. Saint-Germain's merciless demeanor can be forgiven and even forgotten when Cardia's attention (and thus the player's) is drawn back to her primary love interest, but that doesn't happen on Saint-Germain's route when he's the only other person present and she has to come to grips with the fact the guy she has an affinity for just snuffed out her brother's life like he was a mosquito.
Saint-Germain's route gets rather weird and uncomfortable fast and stays that way for at least a couple chapters because everything is a secret about him. All we know is that suddenly he wants to kill Cardia and he's isolated her from the rest of the gang to do it.
He doesn't come off as wanting to go through with murdering her, which prevents him from being completely irredeemable, but he's clearly willing, since there are multiple bad endings where he follows through. Fortunately, Cardia is a fairly active protagonist, so even though she cares about Saint-Germain and worries about what he's hiding, she has no interest in passively waiting around for him to kill her either.
I was relieved when she manages to escape (quite smartly too) and make her way back to the other guys, who are gallant enough to help her figure out what's going on with Saint-Germain and his connection to the secret organization known as Idea (pronounced ee-DAY-ah).
The story that comes out is a bit over the top, but this is where his route gets interesting. In a nutshell, Idea is run by the biblical Eve, who can see the myriad possibilities of the future due to having eaten of the forbidden fruit. To make up for her failures she is trying to guide humanity to salvation by preventing catastrophes that would wipe out the human race. The immortal Apostles of Idea, which include Saint-Germain, are her enforcers who do whatever is needed to prevent disasters, even if it means killing a few innocents to save a greater number of lives.
The example Saint-Germain angsts over in game is his failure to stop the Black Plague in Europe, which killed millions, and he could have prevented it if he had executed an infected boy he instead took pity on.
Cardia, being an artificial life, is seen by Idea as an attempt to trespass into the realm of God. Saint-Germain killed Finis for the same reason and presumably killed Cardia's father as well, though Eve doesn't specify which agent specifically did the deed.
But there's a slight problem in that Saint-Germain has developed feelings for Cardia, so he's having trouble following through with her.
Assuming the right choices are made, Saint-Germain eventually decides his love for Cardia is stronger than his sense of duty and turns against Idea knowing that doing so will cause Eve to revoke his immortality, so he races to take out the remaining Apostles and Eve before his body degrades entirely, since he originally died thousands of years ago.
Naturally this won't make for a happy ending for him and Cardia since Saint-Germain will end up dead at the end of it even if he succeeds. So it's up to Cardia and the rest of the guys to come up with a plan to haul Saint-Germain's butt out of the fire (even if he did try to kill Cardia).
While I generally dislike it when love interests run around with a deathwish, Saint-Germain's torment really sells it. The way things stand at the point he turns his back on Idea, he really can't do anything to earn a happy ending with Cardia. If he doesn't kill her, another Apostle will, so his only option is to stop them, and by betraying them, he seals his own fate.
The situation the count is forced into, where all options are bad, made for a good story, and when Cardia makes a gambit of her own to save his life, her options are similarly either bad or worse.
I did have two complaints when I got to the end though. 1) Cardia and Saint-Germain both get a little ridiculous in their willingness to die for their other half, when it's clear that whoever survives is going to be miserable. 2) Saint Germain's storyline doesn't make it clear why he never tries to kill Cardia in the other routes, since presumably that has been his secret mission the entire time. If there was an event specific to his route that gave him a reason to kill Cardia that didn't exist on others, this would make more sense.
Lupin and Victor have surprisingly prominent roles in Saint-Germain's storyline, likely because Saint-Germain himself is actually a villain for a fair portion of his own route. I was fairly neutral towards Lupin previously, but after seeing how much he stands by Cardia and tries to help her when Saint-Germain is their enemy/rogue cannon I found I really liked him and I was looking forward to his route. I'm glad I played this one second to last.
Victor is always sympathetic to Cardia, but Saint-Germain's route is notable in that it actually foreshadows what's going to happen to Cardia in Victor's storyline. It makes me want to replay Victor's and see if I can pick out just when he realizes she's turning into a poison time bomb.