Well I developed them through roleplays I used to do which are long gone now.
To summarize my Flandre backstory and personality theories. I feel that they originated from Europe (which I am fairly sure is written in the canon theories). It’s been a long time since I RPd but there’s also a belief that Flandre and Remilia were forcibly changed into vampires as kids. There’s an entire fandom video that shows a popular fanon idea that the Crimson Moon changed them and freed them from abuse and imprisonment where Remilia pretty much slaughtered everyone in the house and escaped with Flandre.
I took this idea and ran with it. I felt that Flandre was horribly abused in her human life and, when changed into a vampire, became a bit “Feral” in nature. To those who have made no impression on her, they are nothing but toys for her to play with. She has absolutely no concern about how violent she ‘plays’ with others and seems to be really bored with how no one can actually survive her ‘games’.
This is one thing that I used to get her attached to OCs and develop friendships. OCs who were powerful enough to survive her games, she would take an interest in them. I almost never used her ultimate power AKA “KYUU”. It was only used in certain situations where her life was in danger because I felt that she would use this immediately if she felt scared at all.
That added to the instability and danger OCs faced when interacting with Flandre. They had to be careful not to scare her and make her feel as if her life was in danger.
An opposite to this theory is that Flandre doesn’t actually ‘know’ her games are dangerous and just considers people as disposable toys, completely disregarding them and fueling her boredom. While I think that this theory can be correct too, I felt it removed a lot of potential psychological depth from her so I deviated from this idea.
I took her in a darker direction where she knew she was being dangerous and enjoyed trying to kill others. Canonically, it was shown that she “Kyuus” things out of ‘boredom’ as part of an experiment with a meteor that was crashing down on Gensokyo.
But you really have to write that part of her character out if you want to function in a roleplay environment with OCs. She was initially going to Kyuu Reimu in Embodiment of the Scarlet Devil after beating her but Reimu pretty much weakened her too much that she didn’t bother. Reimu really did anger her though but she calmed down a bit during the dialogue.
The actual story is that Reimu was attacking her while Marisa was dodging her attacks. Which you can see that she takes a liking to Marisa if you beat the game on Marisa’s route because of Marisa’s ability to avoid her attacks and ya know ‘not die’ which entertained her, but she hated Reimu.
Boy pulling this all from memory is rough. I’ll let you read it through and reply and correct me if I remembered a detail wrong. Flandre is a very difficult character to portray because there are so many different possibilities to how she actually is personality-wise. (You ever notice Flandre fans argue with each other all the time in Touhou communities over what her actual personality is? lol)