Friend in disguise.
Friend (友達 Tomodachi) is the main antagonist of the series. He is the mysterious, charismatic leader of an unnamed religious cult and its associated political wing, a Japanese political party known as the "Friendship Democratic Party." He is the father of Kanna Endo.
For most of the series Friend's identity is unknown, although it is surmised early on that he must be one of Kenji's former classmates. He is determined to put an end to humanity using the plans created by Kenji's Youth Group, otherwise known as The Prophecy Book.
Friend's identity is revealed to be Fukubei Hattori shortly after his death. However, while the world mourns, Friend's most carefully hidden accomplice, Katsumata, takes his place. The two Friends are most easily distinguished by their favored masks: Fukubei's Ninja Hattori-kun mask serves as an oblique clue to his identity, while Katsumata replaces it with the now iconic symbol-bearing mask.
The ultimate motivation of Friend depends on the man behind the mask at the time. Fukubei bears a grudge against Kenji owing to an inferiority complex and his own questionable mental state. Katsumata's grudge is much more specific, hailing from the time Kenji allowed Katsumata to publicly take the blame for a petty theft at a local shop, resulting in his being excluded and ignored by the other children. Further, since Fukubei died unexpectedly and the final parts of the New Prophecy Book were written by Katsumata, it is unclear what Fukubei's ultimate goal would have been had he lived.