Complex sentence: Eventually he learns from Subaru that Tsukasa has been online for the last 10 days; he is also the one to discover that Tsukasa in real life is in fact an hospitalized girl, who has fallen in a coma while playing "The World".
Split into simpler sentences: Eventually he learns from Subaru that Tsukasa has been online for the last 10 days. He is also the one to discover that Tsukasa is, in real life, a hospitalized girl who has fallen in a coma while playing "The World".