I selected Impey for my third playthrough of Code: Realize because I was pretty sure I was going to have the least fun on his route and I wanted him to be the hump to get over before my ride accelerated again to the finish. The problem with Impey is that he's not a particular deep character. He's fantastic as a supporting cast member, but the other love interests clearly have agendas that have helped bring them together.
Impey is along for the ride because something of his was stolen and he hooked up with Lupin to get it back, and it's clear that finding the thief is a secondary priority to finding Cardia's father and stopping a terrorist plot, which is more or less the end goal at the start of the game. Aside from that, Impey serves as the group's comedy relief. He's loud, he's brash, he's the wannabe casanova who can't get a date, and he's known for two talents; being a good engineer and being a good cook.
From the moment he's introduced he's constantly hitting on Cardia and it makes it hard for a real romance to build. Their initial conversations are entertaining because she begins the game as a largely emotionless doll so when she shuts down Impey's flirtatious invitations she does so in a hilariously blunt fashion with no harm intended save the truth itself. Even moments when she chooses to hang out with him are phrased along the lines of "Well, I'm going with you because Lupin looks too busy and Victor's tired, so you were the last one left."
Because Impey is constantly proclaiming his love for Cardia, and Cardia is constantly pushing back (even correcting him when he tells one of the villains not to hurt his girlfriend), it doesn't feel entirely natural when she first admits she loves him, which actually happens a lot earlier than in other routes.
There are a couple nudges along the way to make it clear that Impey isn't just blowing smoke, but her admission comes out in a rush instead of a gradual awakening. It's more of a "Wow, I guess I do like you!" Considering that the admission comes under pressure doesn't help either, and Impey is so keen to prove to the other guys that she actually likes him that I can't help feeling bad for her embarrassment.
Also on his route it turns out that he's a vampire, which explains why he can constantly be pummeled by antagonists without getting killed. But even that revelation felt underwhelming. Though it makes narrative sense to explain his physical abilities (and vampires in this world aren't sunlight sensitive), it feels rather tacked on. There's a whole chapter in the shared route that deals with the fallout from the Vampire War, and while Impey might not be interested in revealing that he's a vampire, his reactions to the attempted genocide are muted and easily mistaken or forgotten.
This is a stark contrast to Victor, who reacts fairly noticeably and more often when elements of his personal story are touched on in the shared route, even if he might not explain why.
I suppose on the one hand, Impey is the last love interest I would have pegged as a vampire precisely because he's the butt of jokes, so there's something about breaking stereotypes. I don't see many greasemonkey vampires who are terrible at flirting with girls. But what you see with Impey, is pretty much what you get. Even being a vampire doesn't feel like a terrible secret so much as something he just never bothered to mention.
When Cardia pushes to know everything about him, we get a story about how he became an engineer, and why he wants to go to the moon in honor of an old friend. It's in character for Impey, but it just doesn't feel like it expanded his character as much as Van Helsing and Victor's backstories. Their flashback sequences allow the player to see another side of them, but Impey is still Impey.
His route also hurts for having the least compelling final villain. Van Helsing's route ends with a confrontation against his mentor, who he had previously trusted. Victor's route ends facing off against Queen Victoria, who is diametrically opposed to him on the use of Zicterium, which he created while in her employ.
Impey's villain is Captain Nemo (also from a Jules Verne novel), but Nemo is presented as a comically insane side character prior to the route split, unlike Victoria and Aleister, who are respectively the queen of Britain and one of the highest ranked agents in Twilight.
It turns out that Nemo is the one who stole Impey's anti-gravity gadget and he uses it to fly a gigantic air fortress called the Nautilus with which he intends to punish anyone in the world who does not acknowledge the power of science. He also has a serious case of "Please notice me, senpai" for those who get the joke.
The result is that the final villain is a whackjob (the characters in game actually refer to him as that) with minimal ties to the main story. Though the game tries to shoehorn him into the greater conflict, it doesn't quite work. His biggest connection is that he stole Impey's gadget, which is given short shrift in every other route, and the other two strands are also tenuous. He knew Cardia's father, but there's virtually nothing new to be learned from him, and even though Twilight is working with him, to the point they actually ditched the government, the reason for that is never explained nor is there enough information to guess why they would in the first place, considering Nemo's insanity.
Also, this may be more of a personal thing, but I disliked Impey's need to show off. Cardia is a competent heroine, but if Impey is in the position where he can defend her, he outright refuses to let her fight, even when she says she wants her chance to protect him. Unlike Victor's ending, where choosing to trust him at the final decision point nets the good ending, not choosing to trust Impey earns his good ending, because Impey wants to pull one of those heroic deaths while sending his beloved to safety.
Impey is just very macho about how he wants to present himself to his girl and the fact he doesn't trust or respect Cardia's wishes irritates me.