What everybody seems to not understand is that she's technically both.
In normal day-to-day encounters, she's a quiet shut-in that has trouble speaking to people because of the age gap, mentality of being a vampire, and having very little contact with others aside from the few residents that she interacts with in the mansion.
When the vampire instincts kick in, she takes on the view of a predator, coupled with the world-view and energy drive of a young girl that wants to "play". Plus, her personality, when it's shining in the forefront, is one of a brat. She's shy and introverted in the beginning, but once she opens up it's balls to the wall, go, go, go! The issue is that while she can control her "pop goes the weasel" power at will, she lacks the ability to "stop" her own strength and playfulness (and murderness to the target) so she willfully stays in the basement because it's safer for everyone and she likes it cause it's comfy.
She, like her sister, is a very multifaceted individual that you would want as a friend until you got tired out and had to convince her to not nuke a city, but gods help you if you're an enemy or target.
Come to think of it, for all headcanon of the fans over the decades, she presents more as being ADHD instead of some turbomurderer or a sad little muffin.
When things get serious, though...watch her demeanor change like taking off a mask. That's one thing about "child" vampires: they may act young, but when push comes to shove watch the eyes and body language. You don't live that long without getting wise to the ways of the world. Even Remilia shifts when Ran is talking about the moon.