Haseo was practically made to fit this role. Here's a handy list of parallels.

• Skeith is nicknamed The Terror of Death, as well as The Shadow of Death, and is depicted as a grim reaper/angel of death in its appearance. Each of the Eight Phases are also meant to represent some aspect of humanity, and Skeith coincides with both death itself and responses associated with death and mortality.
  
  
• Haseo bonds with Skeith after a near (virtual) death experience. Technically, this happens twice, since Skeith is a constant presence with Haseo but is unable to properly manifest until certain conditions are met. The first event is when Haseo is killed by who he thinks is Tri-Edge: an attack that should have rendered him comatose in the real world instead just reformatted his computer and reset his character level, thanks to Skeith's protection. The second event is when he's outmatched by an opponent in the game's arena and about to lose, when losing would also mean losing his only shot at getting valuable information. This fear of loss and association with virtual death is enough to finally summon Skeith. There are other events after this as well where Haseo summons Skeith, both intentionally and unintentionally, due to a fear of losing and a threat of "death".
  
  
• Only Epitaph Users bonded to an Avatar can see AIDA, which are invisible creatures within the game world that can possess normal players and manipulate their emotions. They can sometimes manifest as black masses that even normal players can see. Basically, Internet ghosts.
  
  
• Similar to how a geist can protect their host passively by warding off infection/poison/etc., Avatars also allow their hosts to resist attempts at altering their character's data, such as viruses or infection in the virtual world.
  
  
• Near the end of vol.3, Haseo basically "dies" and has to reach an understanding with Skeith in order to "resurrect" himself, so that's also pretty blatant.
  
  
• Less of a real point and more of an amusing similarity, but Twilight is a term of great importance in both .hack and in Geist. In The World, Twilight refers to a great number of things, but all things related to Twilight are of something beyond the normal game that either can't be seen by normal players or that exists in another plane of reality within the game. Twilight in Geist refers to the plane where ghosts exist alongside the mortal realm, something unseen by normal people. Twilight in .hack also mainly refers to an "item" called the Key of the Twilight, and Keys happen to also be a major theme of Geist. Fun facts!!

I could probably keep going, but you get the idea. His history also translates really well to an AU so hey there's that.