The events in this book only partly relate to everything what's going on in the anime/manga.
Keigo knew Touya since they were little, about 6 years old. They had the time of their lives, always hanging out with each other, playing and secretly talking on the phone till really late at night. But as they were 14, or maybe 15 Touya vanished out of Keigos life all of sudden. He didnt gave a warning, nothing and when Keigo heard, that Touya died, he believed it. Of couse he did. There was nothing proofing it wrong so he believed it.
But then, after years, they suddenly meet eachother again. Keigo as the Nr.2 Hero in japan and Touya as a member of the league of Villains.