on avatars, as written by Kuhn and Pi
1: WHAT ARE AVATARS?
Avatars are AI programs derived from Morganna*. The avatars, in terms of the .hack//G.U. games, can only be sensed by other Epitaph Users--player characters with an Avatar program installed on their character data. In .hack//G.U., there are 8 Epitaph Users. Three out of the eight are currently in camp--Haseo with Skeith, the Terror of Death; Pi with Tarvos, the Avenger; Kuhn with Magus, the Propagation.
Even though we're referring to the Avatars as programs, they're a more then programs and are some form of AI themselves. They all show distinctive personalities of their own and talk to their players occasionally. They also provide greater immersion into The World, although not as much as if they were actually there. In exchange, playing The World with your Avatar active is a more draining experience then if you were just playing a regular MMORPG. They also tend to enhance certain feelings, such as rage or passion--and can triggered by the same emotions. The Avatar program also "chooses" a character that they have affinity with. Endrance, another Epitaph User is picked by Macha, the Temptress, the remains of a AI character he had a deep relationship with on in R:1, for example.
All Avatars were originally designed to bring Aura back after she disappeared at the end of R:1, by recreating the parts of Morganna, and when the virus AIDA appeared, they were repurposed to fight data anomalies, since they all had the ability to absorb and manipulate data with Data Drain. Magus can perfectly replicate any data shown to him. Inis can create illusions. Skeith might be able to absorb something where other Avatars couldn't. The game canon is rather vague on this point.
2: HOW AVATARS WORK
Avatar programs can only be activated by the person who has the "program" installed on their Player Character. Thus, only Kuhn can activate Magus and Haseo can activate Skeith. In fact, normal people who don't have the connection to the avatar will find themselves dizzy when they try to log into an account that has an Avatar connected to it, and there's no way to "uninistall" the Avatar program. It will bond to your PC, then it'll be awakened so that you can use it, and after awaking, canon implies that taking it away from the PC would be very traumatic and possibly result in a psychiatric break and or a coma. And if a Avatar is attacked, it is theorized that it will be rendered unstable or permanently unusable and thus be effectively destroyed.
Avatar battles resemble a cross between such space combat games like Asteroid and a beat-em-up like Super Smash Brothers. You dart around the screen and doge incoming attacks, and attack back Avatar Space, which can only be seen by Epitaph Users normally. You beat your opponent and then you drain the program or data away so that the other entity doesn't have it anymore and you do. Avatar powers can be used on people without summoning the epitaph as well, but if the other person can put up a fight, a battle must take place. Only one Avatar (or two, if one is going berserk) to a fight--although, if need be, other Avatar users can "give" up their current power temporarily to another Avatar user to back them up in a fight. This is because Avatar Space starts to warp the minute more the one Avatar is used, and the programing of the World becomes more fragile and prone to breakage.
*: ON MORGANNA AND THE EVENTS OF R:1 AND THE DEATH OF THE WORLD
(If visual data is more of your thing or you want to go more in-depth, you can watch the Terminal Disc, or read a full transcript or a timeline, but here's a handy summary of the relevant events with a few more footnotes of the history behind the Avatars.)
In the first .hack series, much of the mayhem and madness is actually done by Morganna Mode Gone, or Morganna as she was more frequently called. Morganna was programed by Harold Horwick to produce the "ultimate AI" from the actions, memories and personalities displayed by the players of the World. He considered Aura to be his "child" with Emily Wielent, a poet who had a deep love for and whose works he integrated into the system. Morgana did this, but eventually discovered that once she created the AI, known as Aura, that she would be discarded and have no purpose in the world anymore. She created the Epitaph of Twilight, which was full of her rage and disappointment to counter this. Eventually, as Kite and others fought to save Aura and the events of the World progressed, she began to malfunction more and more, causing data errors and bugs to occur in the World. She also began to place parts of her own programing and that of the Epitaph of Twilight into the 8 Phases, which collectively began to be referred to as the Cursed Wave. With the help of Helba, a powerful hacker and his bracelet which had the power of Data Drain Kite managed to destroy the 8 phases with the aid of Helba and the rest of the .hackers, and then to defeat Cubia, the anti-existence of the bracelet.
After the "death" of Morganna, there was a brief renaissance in the World, where Aura acted as the main administrator and programmer and created a child, Zefie. The internet, connected to the CC Corporation's severs as part of the backbone of the internet begins to depend on Aura's presence to keep its vitality going. And then, one day, Aura disappears. This causes a panic in CC Corp's administrators, businessmen, and network mangers, and the Salvation Plan is made. Including Jyotaro Amagi, 19 year old genius programmer, and Jun Basoya, Pi's older half-brother and AI psychologist, it is created to make a new Ultimate AI. They track and gather the remains of Morganna and put them into PCs so that they can be under the control of the CC Corporation administration. They also identify 8 players that would be ideal to use these PCs. However, frightened by the possibilty of creating another anti-existence, Basoya steals the program of Tarvos. Amagi, anticipating this, has programed a dummy Morgana Factor into a PC. They run the RA, or Restore Aura program with disastrous results. All 8 players fall into a coma. Much of the World's location data is destroyed. Many of the Morgana Factors disappear from the World's servers. Amagi is driven insane and sets fire to the servers containing the World. CC Corporation decides to cut their losses and release R:2, or the second version of the World, which is a combination of some of the old backbone of R:1 and new code.
Basoya, on the last day of R:1, logs on as his private character and is told by Zefie that Aura has abandoned the World shortly before disappearing herself. Jun, no longer a CC employee, continues to watch the world for the Morgana Factor. The Phases begin to show up again, starting with Ovan's appearance with Corbinik, of the Rebirth, and a mysterious knowledge of the World's secrets and something in his right arm that shouldn't exist in the World. Frightened by this, he mails his half-sister, Reiko Sakai the Terminal Disc, which contains a summary of the events of R:1 and the End of The World, and the Morgana Factor of Tarvos. He dies shortly thereafter. She quits her former programing job and gets hired as the a programmer on the restoration project of the World, and creates the PC known as Pi to load with Tarvos's data. And then, .hack//Roots begins.
END SPOILERS.
3: ON AVATARS AND CAMP
In camp, we figure that anyone who has a connection to computer data-such as Wakaba, Dari or the DDS folks-can see the avatars (and if you could comment here to note that, it'd be a big help in keeping track of who can do what). If you have a mystical or a magical connection, too, we're fine with it pinging you. If you want to know what the Avatars look like, here's a picture of Haseo's, Pi's, and Kuhn's avatars.
If you have a question on whether your character can see the avatars or something else--drop a question here, we'll do our best to clarify.