If you see the fanbase, you'll notice that Flan is often paired up with Marisa and Koishi. I really don't know why for Marisa, plant your theories! Now as for Koishi: their both Ex-bosses, their both stronger than their sisters, and their both crazy. It may make sense, but I disagree. Also, she is sometimes, sometimes, paired up with Nue. Obviously because they have the coolest looking wings in the game and they are both Ex-bosses when you think about it.
People put Flandre in 2 categories: Adorable and Insane. It is actually hard to tell whether she has the nature of a calm little girl or a mentally unstable demon who could crack at any moment. Perhaps both at some times. We'll only know if they put Flandre in another game...and she gets a role. Plus, a lot of the fanart portrays Flandre outside. Yeah you decide.