Empathy Pet

A cute little pet sidekick whose emotions and reactions are closely synchronized with their keeper. The pet serves the function of the traditional zany from the Commedia Dell'Arte, aping their human's reactions, and frequently revealing attitudes that the main character may be keeping hidden.

When alone, the character could also talk with this pet, treating them as a separate person with the knowledge that would make the pet an extension of the owner's personality.

Not to be confused with a Morality Pet. Mons may act in this role, especially the main character's.

Examples of Empathy Pet include:

Anime and Manga

Guvava from Macross 7 (→ Mylene)
Q-chan, a holograph resembling a floating blob, from Vandread (→ Misty Cornwell)
Boota from Tengen Toppa Gurren Lagann.
Melpo in Yes! Pretty Cure 5 GoGo. (→ Syrup)
Xiao-Mei from the manga version of Fullmetal Alchemist (→ Mei-Chan, to the point that Xiao-Mei also mimics the girl's movements)
- Black Hayate → Riza Hawkeye. Like his master, he is quiet, cute, and has a whole lot of persistence and courage packed into a small frame (witness his attack on Gluttony when the latter has Riza cornered).
Puu from Yu Yu Hakusho (→ Yusuke). Well, he IS a representation of his soul...
Maya from Azumanga Daioh, though he shows up much later in the series than most (→ Sakaki)
Happens in the Pokémon anime a fair deal, but the Mime Jr. species is particularly notable - its very nature means it exactly duplicates the gestures and expressions of its owner. The fact that one of the main characters happens to own one helps show this.
Marguerite's snake in One Piece, and any other amazon's Snake Weapon.
- Also, Chimney's Bunny/Cat thing Gonbe. He's probably the most literal example, pretty much being a second Chimney in every way. He mimics her movements, facial expressions, and even his growls/mews/whatever-the-hell sound he makes are the same number of syllables as every one of Chimney's statements.
Boogie-kun from Karin (→ Anju) clearly reflects his master's thoughts, even the ones she resents having.
07-Ghost takes this trope to a whole new level: Not only is Burupya Teito's certified empathy pet as he is also Mikage's incarnation. Thus making this cuter than cute dragon a mix of pet and best friend, bonus points for the Heroic Sacrifice involved.
Yumekichi the monkey (→ Keiji) in Sengoku Basara often mimics his master's actions and serves to reflect his true emotions when he enters Stepford Smiler mode.

Comic Books

Old Lace from Runaways is telepathically bonded to Gert and after her death to Chase.
Doctor Nemesis of X-Club gets stuck with one after a telepathic starfish bonds to his head. Said starfish broadcasts his thoughts, which he first discovered when the starfish revealed that Nemesis often admires his co-worker Doctor Rao's "shapely behind" (while Doctor Rao is standing right next to him). The starfish later reveals that Nemesis is jealous of Cyclops' cool costume. It also reveals that while Nemesis doesn't think too highly of his co-worker Jeffries' scientific methodology, he does crave his friendship. Nemesis understandably wants said starfish gone as soon as possible.
- But then at the end of the series he keeps it because he "[gets] lonely without it."
In their definitions Ryuu-Nekos from Dreamkeepers are discribed as having this as an attribute when they bond with or are tamed by their owners in addition to the personality quirks already present in their breed.
Hal Jordan had a pet alien starfish named Itty in his space trucker days.
Kitty Pryde's dragon Lockheed.

Film

Salacious Crumb from Return of the Jedi (→ Jabba; for those who don't recognize the name, he's the little "Oh, OHH hohohohohoho!" guy)
Disney Animated Canon examples:
- Abu from Aladdin (→ Aladdin)
- Rajah, also from Aladdin (→ Jasmine)
- Meeko and Flit from Pocahontas (→ Pocahontas)
  - Pocahontas is interesting because the villain, Ratcliffe, also has one, a pug named Percy, who does a Heel Face Turn and joins the good guys.
    - Percy, while he is meant to be an Empathy Pet, doesn't exactly share much in personality with Ratcliffe. All they really have in common is a preference for the finer things in life.
- Djali from The Hunchback of Notre Dame, who was actually from the original book (→ Esmeralda)
- Pegasus from |Hercules, who is also a mythic character, but not originally from the Hercules mythos (→ Hercules)
This happens in other "Disney-like" animated films too:
- Pooka from Anastasia (→ Anastasia)
The dog and cat from Swing Time (→ the Watsons)

Literature

Although it would be in the worst taste to label them as "pets", Dæmons (animal avatars of a person's soul) in the His Dark Materials trilogy often serve this purpose to their person.
In the Honor Harrington series, treecats can be argued to be this on occasion, being small, empathic animals, who just happen to be sentient and suprisingly protective of their two-legs.

Tabletop Games

Pretty much the entire concept of Familiars from Dungeons & Dragons.

Video Games

Alice's dolls in the Touhou series.
The dog in Fable II can be trained to act like this.
Rhyme from The World Ends With You during the second chapter (→ Beat).
Nanomachine from the Galaxy Angel gameverse (→ Vanilla)
Cupil from Skies of Arcadia, who also acts as an Empathic Weapon (→ Fina)
Popoi when he's with Ms Accord in Puyo Puyo

Web Comics

The cat on Ian's head in Mac Hall.
Chester in CRFH (→ Dave). Of course, he does have half of Dave's soul, making it a Justified Trope.
Lita, Zii's cat in Ménage à 3, pictured above.

Western Animation

Rufus from Kim Possible (→ Ron)
Opalescence from My Little Pony Friendship Is Magic (→ Rarity).
- Tank (→ Rainbow Dash)
- Winona (→ Applejack)
- Philameena (→ Princess Celestia)

Real Life

The concept is Truth in Television; some animals react very markedly to the emotions of their keeper and sometimes others. Though like everything else, fiction obviously exaggerates the effects.