This is a topic for discussing characters from other visual novels (or just other media in general) that are similar to characters from Key, and arguing how that character could be perceived as a ‘rip-off’ in one regard or another. This also holds for if you want to argue that a Key character ‘rips-off’ another character. Naturally, hide any spoilers regarding each character and point you present.
My example is how many aspects of Sachi from the Grisaia series are taken straight from Kotomi Ichinose. (Spoilers for The Fruit Of Grisaia and CLANNAD)
First of all, they’re both very obedient people who are presented as having a lack of conventional social skills. They take everything they’re told literally, and are both very intelligent characters. If it was this alone, I would just take that to mean they have similar character traits, but what stands out to me is Sachi’s backstory and relation to the protagonist. Both Kotomi and Sachi knew their novels respective protagonist as a child, before the events of the novel. They can both remember the protagonist upon meeting them again, or at least soon after, but the protagonists can’t remember them until later events. Both Kotomi and Sachi were annoyed during their childhood that their parents couldn’t spend much time with them because of work. Both Kotomi’s and Sachi’s parents die around the time of their respective birthdays (Sachi’s mum actually lived but is in a coma, but still – minor detail). Both characters lose contact with the protagonist during these events. The novels respective protagonists both remember their childhood because of where they use to spend time with those characters (E.g. Tomoya remembers his childhood with Kotomi when he goes to her house, Yuuji remembers when thinking about the park he use to play at). And both the characters are somewhat traumatised by the events of their parents death as a child, and their routes are about the protagonist helping them to accept what happened. To me, there are so many details that I can’t help but consider Sachi’s premise as a Kotomi rip-off – however, I will admit that regardless of this I do still like Sachi as a character because besides the backstory and her relation to the protagonist, what she does in Grisaia is very different from Kotomi, but this still stood out to me a lot when initially reading Grisaia.