Haseo's player has actually been in the series a while. He was originally the PK Sora from the series .hack//Sign, but fell into a coma from the events in the series. After regaining consciousness, he eventually started playing "The World: R2" as Haseo and was PKed by Iyoten and Asta his first time logging in, though Ovan stepped in to save him. Shortly after that, and with persuasion from Shino, he joins Ovan's guild "Twilight Brigade". He and Shino become friends, though with Shino's single-minded quest to find Ovan in Roots (and after Redemption) it is questionable whether the relationship was one-sided.
Shino was later PK'd by Tri-Edge and her player fell into a coma. This led to Haseo becoming the dreaded PKK "The Terror of Death" as he hunted to find "Tri-Edge".
Ovan directs Haseo back to the Hulle Granze Cathedral, leading him to fight Azure Kite and is Data Drained by remaining between he and Ovan, reformatting his computer and stripping his character of items, abilities and back to level 1. From that point Haseo begins the G.U. trilogy.