Mirror spirits

Mirror spirits are a type of toolspawn born from mirrors hence the name.

Appearance and anatomy
Corporeal mirror spirits use the mirror as their own body, projecting faces it has seen before in its glassy surface.

Incorporeal mirror spirits have a humanoid form separate from the mother object that has glassy eyes and hair and pale skin. They can mimic a certain person's appearance almost wholly with the exception of the eyes remaining ever glassy.

The mirror is simply made of the reflective glass and its framing that can protect it from damage.

Manipulating light through reflection and refraction
A mirror spirit can manipulate light through reflecting and refracting. This allows them to create optical illusions out of thin air. Through practice, a mirror spirit can manipulate light to the point of it crystallizing and turning into barriers.

Mimicking one's appearance and personality
A mirror spirit can mimic one's appearance almost perfectly. Typically, the mirror spirits copy those they immediately see, but they can copy someone's appearance almost fully, except for the eyes which, for some reason, they can't replicate. Mirror spirits that have aged can possibly replicate one's personality as well.

"Seeing" the past
Once a mirror spirit is old enough, it can begin to see glimpses of someone's past if someone looks at their mother object. How it does is unknown but the Youkai Coda speculates that:"'The eyes are the windows to one's soul through which the spirit of the mirror can see the raw emotions of the man. By looking at the mirror, its spirit can see through the eyes of the one who gazes at it. It feels what the untamed soul feels and has felt, and it thinks what the unlying soul thinks and has thought. Perhaps, through this trickery of light, the mirror spirit peeks through the looking glass of men (that is, his eyes).' - Shirotake no Kyoui on the Judgment of Sazanka no Hoshizora, Youkai Coda, 1860 TE" Since mirror spirits are toolspawns that are related to souls, this hypothesis in looking into the past through the soul of a human is the most widely accepted hypothesis in its hindsight abilities.

Vulnerability
The mirror spirit's mother object is vulnerable to damage and so is the toolspawn itself. Upon birth emerging out of the mirror, it will be protective of its mother object, going so far as to stow it away somewhere else. This makes most mirror spirits feel obligated to find a place to stay. An excerpt says:"'We [mirror spirits] don't like the concept of our mirrors being smashed into pieces. You humans couldn't possibly understand since your flesh and bone are strong. Our glass is fragile. A strong tap at our glass feels like [we're] being pushed violently. [...] Imagine breaking every single bone in your body. That pain is what we feel if our glass pane cracks. Such a horrifying concept...' - Interview of Mary Stansmuth, 2005 TE"

Services towards humankind
Mirror spirits are usually associated with royal families in Eimbus, steampunk Litero, and Vernicht during the middle to late 1700s. This could be because of royals being tidy, diligent, and scrupulous towards their possessions, mirrors being one of them. This allows the mirror to birth mirror spirits and provide services to the families. Many tell of the mirror spirits acting as wonderful maids and butlers, with one royal saying:"'A royal family can only prosper further through the guidance of a mirror. One could boast of their precious technological advancements, and another could scoff at it and boast of his own achievements in physical form, but the mere mention of the mirror being your most prized servant would make the sternest of royals gasp in awe. I can tell with my own experience with my own [mirror spirit] and his obedient demeanor, diligent personality, and on-point advice!' - anonymous Literoan royal, ~1700s TE" With royals being less of an influential power in the modern age, mirror spirits are growing less and less impactful on acting as maids and butlers. However, mirror spirits continue to flourish in Vernicht where they act as aides of judges during convictions thanks to their ability to see into the past. Their brutally honest attitude has given them another role to keep in life, making Vernicht a "haven" for mirror spirits. Shirogane and Kurokau shortly followed suit. A mirror spirit acting as a judge has said:"'Humans barely know and rarely tell the truth. Our prying into the true nature of the convicted and seeing what they really saw has helped humans continue to pursue the truth. In my years of being a judge, I can see the humans seeing the value of truth more and more. I believe we have done this improvement to them.' - anonymous Vernichti judge, 2015 TE"

Nourishment
Mirror spirits, especially corporeal ones, don't need to eat. Incorporeal mirror spirits don't either, but they will "eat" one's emotions if necessary.

The Mirror-breaking Incident
There have been reports of "mirror spirits" causing curses and ill-mannered actions towards humans in Eimbus, particularly in the late 1700s. This has urged many a household to crush their mirrors into pieces, especially those afflicted with these malevolent spirits only to find that it didn't work at all.

Only when the spiritual priest Hanshisha Reishoukan intervened did this incident finally close down. As it turns out, a previous incident has occured that made many human spirits restless, turning into poltergeists and causing malevolent pranks everywhere.

The incident still caused a decline in mirror spirits everywhere with people opting to purify mirrors from time to time in case a poltergeist possesses their prized mirror. This also led Eimbus to ban mirror spirits altogether which is still in place today, explaining the lack of the toolspawns in Eimbus.

Trivia

 * In Monteverde where still lakes exist, the water acts so much like a mirror that mirror spirits can come out of them. Unlike most mirror spirits, these spirits are rabid in nature and are more powerful and unrestrained compared to their incorporeal counterparts. For more info about this, check out the Hydrospecular Phenomenon.
 * Mirror spirits have told humans how emotions taste with joy tasting like a mildly sweet cheesecake and fright tasting like a roast pepper steak. Oddly enough, this "taste" of emotions can also be observed in other toolspawns like faceless maskmen, sentient instruments, and living dolls.
 * Mirror spirits are based off of the Japanese youkai ungaikyou (a sentient mirror) and the concept of a doppelganger.