Lewis Carroll's Looking Glass is a large mirror, taller than average human height, surrounded by an ornate, gilt frame.
The mirror may serve as a gateway to an alternate dimension, which is how Pete was able to play ping pong by himself.
Background[]
Lewis Carroll, born Charles Lutwidge Dodgson, wrote stories of a wonderland that could be entered through a looking glass in his famous works Alice's Adventures in Wonderland and Through the Looking-Glass. Although commonly thought to be fantasy or fairy-tales, the stories of Alice's trips to wonderland were instead a chronicle of the descent into madness of a woman by the name of Alice Liddell. In his stories he described a mirror that was at one point owned by Alice. Warehouse agents tracked down Alice, who was committing a series of murders at the time, and trapped her inside the mirror, though not before she killed one of them. However, it's unknown exactly how they trapped her in the mirror. The publicly known fantasy stories were Warehouse fabrications to hide the truth.
In its storage location prior to the temporary release of Alice Liddell's psyche, a label on the mirror read:
MIRRORCHARLES L. DODGSON AKALEWIS CARROLLFILE BLOCKB732 . 3 MRS . 028
An even older hand-written paper label attached to the frame at the back of the mirror simply reads "MIRROR - Formerly owned by Chales L Dodgson, A.K.A. Lewis Carroll 1832 - 1898" but does not appear to contain any location codes as with the newer label. The mirror is now housed in an area of the Warehouse called the Dark Vault, where sensory-activated or otherwise extremely dangerous artifacts are stored, to make sure Alice would stay in Wonderland.
How it Works[]
It is unknown exactly how the mirror works, though it does sometimes simply reflect whatever is standing in front of it. However, according to Warehouse records, the mirror originally belonged to Lewis Carroll and only after the mirror crossed paths with Alice Lindell did she begin her reign of terror. It's probable that she used the mirror to kill her victims somehow, or the mirror made her kill those people; either way, it's still an artifact.
When human, that image can frequently operate in a manner that appears autonomous. Pete frequently used the mirror for entertainment, playing ping-pong against his own reflection, though exactly how that image was created and how it continued to operate by itself remains unknown. The only clue to its operation comes from Alice Liddell when Pete confronted her in the Las Vegas hotel room. According to her, nothing came out of the mirror without something else going in, and this assertion held true when she was retrapped in the mirror and Myka's consciousness was set free.
What is known about the mirror's operation relates to interaction with other artifacts. When interacting with the Studio 54 Disco Ball, a process Artie calls "optical divagation" occurred. Artie further describes the process as "kind of like an optical flare after a photo flash." Although he later says he really doesn't know how the mirror works, he initially speculated that the blast from the disco ball "fritzed the mirror's primary reflective capacity and then it probably clipped part of Myka's psyche and fused it" into the mirror. Even though Alice Lindell was in Mykas' body, Alice Lindells' reflection stilled appeared in mirrors she looked in.