Macha

Pre-Games

Originally appearing as Macha in the anime .hack//SIGN that leads up to the events of the game, Macha appears as a small colourful cat. She is believed to be a hacked character by the Crimson Knights as such an appearance is impossible, but in reality, she works for Morgana and can sometimes be found accompanying Tsukasa. 
 
Macha starts to show signs of emotion when an injured Tsukasa gives her some aromatic grass, and later acts against Morgana by protecting Tsukasa from his Guardian, and appearing to be dissolved into data. Since Macha isn't a player and cannot fall into a coma, moreover being a part of the system herself, this is not the end of her appearances.

First Game Series

 (The following contains plot spoilers from .hack//INFECTION, .hack//MUTATION, .hack//OUTBREAK, and .hack//QUARANTINE)

As Mia

When introduced as Mia in .hack//INFECTION, she appears as a very mysterious Blademaster.  Her appearance is very unlike all other player characters in the R:1 version; she takes the form of a tall purple furred humanoid cat with long rabbit like ears and a long thin tail; in many respects, appearing like a taller version of how she appeared in the anime.
 
She has a fondness for aromatic grass and can normally be found alongside her friend, Elk the Wavemaster. Personality wise, she is a very blunt, at times known for speaking in riddles, and rarely raises her voice. 
 
She is the only person other than Kite who can see his bracelet that provides him with his hacking powers, which he is aware other people should not be able to see. She is interested in him for this reason.
 
When the phases start being defeated leading to complications on the servers, she becomes badly affected by it, being a product of the server herself. On various occasions, her voice doesn't function correctly (Elk explains this away as being a microphone problem), walking in some kind of lethargy, and barely appearing to recognise her friends. Defeating further phases leads to her changing again. 
 
After defeating the game, Macha can potentially return in her "Mia" form if the player takes up the challenge.
 

As Macha

As the sixth phase, Macha's appearance drastically changes in a way that makes her look less like a character and more like an enemy. 
 
She is known as Macha, the Temptation, and as such her skills operate around charming her opponents. The party have to defeat Macha, much to Elk's sorrow, in order to fix the issues with The World.
 
She was an A.I. programmed to study love and temptation.  She is the only phase to show any sign of affection for humans.  

.hack//G.U

She would later become Endrance's avatar.