Head canons for Flandre will be updated as I come up with them.
Fiercely loyal to Remilia and would do anything and everything for. That involves slaughtering anyone who would do harm to her, to pretty much staying passive and remaining in the basement whether she wanted to or not.
Takes playing to the extreme if allowed, also destroys all of her belongings out of boredom rather easily.
If allowed out of the basement, Flandre is rather tame and docile, never really seeking the need to leave the mansion unless her sister allows it. [see the first head canons for more specifics.]
Is rather naive and new to many things, such as sympathy for others and the world outside of the confines of the basement. Often spends her time in the library, reading up on things to impress Remilia with when she comes upon her.
Despite if she’s let out of the basement, the residents of the mansion keep her way way of in taking blood into her food a secret, so she doesn’t randomly go out and start feeding on humans or other people within or out of the mansion. She consumes food only prepared by Sakuya still, but occasionally rummages the fridge for sweets when no one is around.
She is also still rather unstable emotion wise, if stimulated too much by external stimuli. [ie, people that would probably invite to play with her or threaten her.] And often takes it to the extreme, though she is learning to be wary of her emotions...for the most part.