Kurusu
About
His real name is unknown. Kurusu is an enigmatic, emotionally-detached and childish person who appears to receive great pleasure in killing the infected. He is first seen in a video where he claims that although he's tried to kill his mother dozens of times, she always came back to life. Many people on the internet have gone on to form a quasi-religious cult around Kurusu, proclaiming him as their prophetic leader in the new age that will emerge from the disaster; however, he doesn't seem to care about this at all and is only interested in killing.
Later on, "Kurusu" is a term used to refer to the Japanese people who manage to fight off the infection.
After a fight between him, the Naked Emperor and Takashi, they are all assimilated into a single entity that can switch appearances according to whichever personality is in control. When the female supply squad member in Tokyo is also assimilated, the Naked Emperor shows her Kurusu's true form, visible only when his personality is "asleep": an old, frail man that couldn't move nor talk since childhood, and only stayed in bed.