WMG: Bubble Gum Crisis 2040

The cause for Sylia's violent mood swings is an imbalance in her brain chemistry, courtesy of her father's unethical experimention on her as a child.
Let's face it, having a self-replicating bioengineered construct growing in your brain and then surgically removed while a child is bound to leave some side effects. (This also explains the medication that she is shown to be on as an attempt to compensate for this.)

Sylia is the daughter of Rally Cheyenne from Silent Möbius.
Those two share a rather uncanny resemblance (ok, mostly the white hair) and affinity for technology. This conveniently explains the slightly organic nature of Boomers and their shapechanging abilities if we assume that Rally's weird half-alien biology is their basis. Rally's also around the right age since she'd be about 50 in 2040 if she was alive.