Finis's story is definitely one that would have been saved for a fan disc since it's not romantic, but it's also really good, and if you have unanswered questions left after ~Guardian of Rebirth~, this likely has the answers. Also, because this deals with the creation of Cardia and Finis, there is a lot of overlap with Lupin's ~Guardian of Rebirth~ route, but in this case it's understandable. There's no way to tell Finis's story without also going into the purpose of Code:Realize, and this time we get additional insights into the same events.
In ~Guardian of Rebirth~, we learn that Finis has been trying to get a hold of Cardia since the beginning of the game. He was the one who sent the order to have her brought from her mansion in Wales. She's a necessary keystone for enacting their father's plan, Code:Realize, which will resurrect him as a sort of machine god who intends to plunge the world into everlasting war to advance science. (War tends to push along scientific progress, so the base logic is sound, if inhumane.)
But Cardia refuses to go along with this. Because she was rescued/kidnapped by Lupin and company who treat her like a person, she's gained a sense of self and has no interest in being turned back into a doll.
Finis's story takes pains to show the parallels between Cardia and Finis as created tools for their father, except that Cardia had the benefit of being among people who loved her, and Finis spends his days enacting his father's plan and never living for anything more than his purpose. When we first meet Cardia, the only things she has on her mind are her father's last words to her. Even outside of this particular route, we already know Finis works so hard specifically for his father's approval, which is sadly an approval that will never come. Isaac views thanking Finis as the equivalent of thanking a screwdriver for doing its job.
It's not until Finis's story though that we get to see why Isaac's view of his homunculi is so dim. Considering that Isaac's first words to Cardia are out of consideration and that he named the two after his deceased children, it came as jarring that he was so callous to them during the climax of Lupin's route.
We finally get to see Isaac's pain, as he starts with Finis and tries over and over to recreate his son by implanting his memories into a homunculus, but each time the homunculus wakes and sees him as a stranger. Though the many iterations of Finis try to become the Finis of their collective memories, it only serves to shove them further into the uncanny valley and make Isaac realize that they can never be his son. This is why Isaac doesn't love Finis and never smiles at him no matter how hard he tries to please him.
Realizing that he needs to do something else if he wants to recreate his family, Isaac goes on to form his Code:Realize plans and creates Cardia (meaning that she's only Finis's older sister because that was the case for the original Cardia, or because of her physical appearance, and not because she was created first). While she is still sleeping, Isaac realizes that she will probably turn out the same as Finis when she wakes, and because of the poison in her body, he would never be able to hold her like his real daughter. So Isaac gives her those parting words from the opening of ~Guardian of Rebirth~ about how she shouldn't fall love because she is a monster and it would only cause her pain.
Since the transmutation from Horologium to Philosopher's Stone was going to take time, and it appears that Isaac knew he didn't have many years left in his life, he made plans for his resurrection, which was a nice touch. I assumed when Isaac had disappeared he had been violently killed somehow, but it looks like age and overwork just caught up with him. And given Cardia's poison build-up, it made sense that he would leave her in his old abandoned home rather than underground in Twilight's headquarters until the Horologium was sufficiently processed to be usable.
The Finis we know is one of many, each connected through a main body "computer" (for lack of a better term). They all share the same memories, and in a sense they're all interchangeable. If one dies, another takes his place.
Cardia finds this horrifying, believing that each Finis by rights should be an individual life. They're more clones than remote controlled dolls.
His route plays out much like Lupin's with the attack on St. Paul's Cathedral and the Nautilus appearing over the city as Code:Realize is enacted, but eventually one of the many Finis clones becomes marked as an individual. This one is discarded and cut off from the main body as Isaac considers it an acceptable loss to get an Apostle of Idea off the Nautilus at the same time. However, this Finis survives the fall and is found by Lupin and company, allowing him to see the kind of life that Cardia had been living.
Other things things of note:
I suspected during Victor's route that there was a new translator working on this one because his Japanese nickname "Fran" shows up once in his route and on the save files for his After Story. There's no doubt to me that the new translator was also working on Finis's route because after using Apostles of Idea all throughout ~Guardian of Rebirth~ and every other route in ~Future Blessings~ we suddenly get "Idea's Apostles" and the term is used consistently throughout Finis's route. It's essentially the same thing, and it's not a "wrong" translation, but it was annoying because its consistency in that one route broke with the consistency in all the others.
We also get introduced to a new Apostle, Hansel Hexenhaus, who is the Hansel from the Hansel and Gretel fairy tale. The soul of his sister Gretel is bonded to his weapon, which is described as a "fork" but that doesn't really do it justice. That thing is more of a trident or ornate battle pitchfork, if you have to keep the "fork" part in it.
I suspect Hansel was created in order to have a different representative present for the finale. Saint-Germain is difficult to use because his character is already established (and besides, he canonically kills Finis on every route, which another Finis is unlikely to forgive without time and distance) and Guinevere is too militant. Hansel, with his brother and sister backstory, is able to be sympathetic to Cardia and Finis, as well as to justify sparing them once Isaac is dead. It's also amusing that he's off-kilter enough that Saint-Germain and Guinevere question his deployment.
Though I'm not entirely sure he was necessary, I did like getting another Apostle. Since there are only thirteen of them, they're a small enough group that they should be individuals rather than random NPC faces.
Lastly, I like that Cardia and Finis go back to Isaac's old house in Wales together and work on rebuilding it to be a proper home. After everything, there's only one Finis left now and the main body is gone, but the Finis that remains is now a real person.