Billie doesn't just sing the blues, she lives the blues. Billie has a good heart, but she can be a total downer. She always sees glass as half-empty (even when it's full), and doesn't like to try new things. Billie has a big-sister relationship with Ruby, and admires her enthusiasm. Billie sings and speaks in a rough and breathy alto. Her catchphrase is "it's enough to make a girl feel kinda blue€_", typically before breaking into song.
LEDs are two-terminal electronic devices, and are a special type of diode. The first modern LEDs were created by engineer Nick Holonyak in 1962 at General Electric. When electrons cross part of the diffusion region of a diode (an area called the band gap), a photon is emitted. The size of the band gap determines the wavelength (color) of the photon. LEDs are available in many wavelengths ranging from infrared to ultraviolet.
Billie the Blue LED - Circuit Playground Plushie (0:04)