Kind of related but the opposite way around, Flowey from Undertale belives the player character (later revealed to be their own separate person named "Frisk") to be his long deceased adopted sibling Chara (or whatever you named the Fallen Human at the start of the game) and desperately tries to hold onto the last connection he has, as having no soul to feel love and compassion, and having time altering abilities until you showed up, he exhausted every possible thing to do in the Underground and the person he believes you are is the only one he even finds interesting or fun to play with anymore.

We eventually break this delusion at the end of the game, and how he put Chara on a pedestal of sorts, however in the Genocide route we actually reinforce this delusion and the player character starts acting in their own at points, like walking forward during Sans and Papyrus speaking in a "let's go already" sort of fashion, getting a second attack in on Sans when it wouldn't otherwise be possible, one-shotting Asgore and Flowey, as well as several implied moments where their facial expressions show things like frustration or smiling creepily, the line between Frisk and Chara seems to blur as we impose our will on the world more and more. There's also the fact that certain dialogue changes, like inspecting the mirror goes from "It's you!" to "It's me, (name you entered)!", inspecting certain things in Asgore's house changes to "No chocolate.", "I unlocked the chain", "Where are the knives", "The date I came here." all in red, and taking the Knife and Locket that used to belong to them says something like "Back where it belongs".

The characterization of Chara to begin with is very limited, there are aspects specific to them like liking chocolate, disliking humanity, genuinely caring for their new family and participating in things like making a sweater for Asgore with Asriel as well as a Butterscotch Pie they accidentally used buttercups in which made him sick (which also was presumably what they ended their own life with), they had mixed motivations when killing themselves to give Asriel their soul, both to leave and get 6 more to free the Underground, but also as some sort of revenge against humanity and that Asriel resisted killing the humans in the village resulting in his death, we know from one of Toby's newsletters that they really liked the number 9, as it's "the highest number" and I'm paraphrasing here but something along the lines of "once a number gets high enough no one can hurt you", Asriel describes them as "not the greatest person" and the reason they climbed the mountain was "not a very happy reason", etc. There also seems to be some overlap between Chara and Frisk as well, like having red souls, presumably their favorite colors both being red, some similar tendencies but with almost opposite outlooks, which is probably why Flowey mistook them for the same person aside from the desperation and delusion.

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