Yggdrasil no Kajitsu

It's the near-future, and online gaming is all the rage! Teenage gamer Ko has more of an edge than others, since his father works for the company that produces the biggest online game out there... at least, he had an edge, until someone hacks his account and starts playing his character! Ko is determined to find out who's been logging into his game, but he and his friends may have found more than they bargained for as they delve deeper into the fabulous world of Yggdrassil...

(Source: Amazon)

Yggdrasil no Kajitsu had two volumes published in English as Yggdrasil by Go Comi from July 1 to October 1, 2008, before the publisher became defunct.