what @Elfenlied1012 said.
Great topic to own. But there are some inaccurate points. One (this is from what I know) SIGN came out wayyy before any of the games back in 2002 and both were only released months apart. One was a novel the other a show (I believe).
Kite & Aura never had a relationship. The game introduced Kite which was sometime after Tsukasa awakened Aura. In the game Aura met Kite and started their story. Then another person won the "Kite" character through an event, coincidentally enough the people playing the Kite & Blackrose characters were previously logged into the world some time in the SIGN series. So in LEGEND OF THE TWILIGHT the person playing "Kite" already knew Aura.
.Hack is definitely a long series to keep up with. SAO is rather short & to the point but skips many details in the characters which doesn't make the audience feel quite connected. Where as .hack has MANY stories & views because the epic novels, games & shows are about everyone. Not so much as just ONE protagonist. It creates a multi-fascinating view on different stories each person in THE WORLD experience. Which some how or another, at least one person can relate to at least one character (no matter how "main" the character is). I hoped this helped abit ^.^