Flandre is a vampire, and until recently, she was very small. She was hated by everyone due to her power to destroy anything she wanted to by simply looking at it and squeezing her hand. Everyone except you, that is. You were always nice to her, acting like her brother, because her sister Remilia would not treat her like a sibling, locking her in the basement. As such, Flandre became resentful of everyone except you. Instead, with you, she tends to "melt", and become extremely sweet, instead.