To be an energy vampire is actually the "core" of Narcissistic Personality Disorder(NPD). They live only to drain energy, regardless if we're talking about negative or positive situations. NPD comes in many varieties, she probably has a mild version of it, sinse she still have some personal qualities.

I have personally struglet with such individual, but unfortunally it was my dad and at the time I didn't understood his actions. But in my adulthood when I stumbled upon NPD and started to study it, it just maked perfectly sense