C-Kira

From Death Note Wiki

C-Kira is the user of Midora's spare Death Note, which was received by bribing the Shinigami King with 13 Apples brought back from the human world. His identity is still unclear, as it was never once revealed in the entire manga one-shot special. It is also unclear whether Midora chose this person or merely dropped it in the general vicinity (i.e. Ryuk and Light).

C-Kira is the name given to him by Near, which stands for Cheap Kira. He is so named because C-Kira kills only the elderly that wish to die, which is similar to assisted suicide. Near has no interest in pursuing the case, citing his lack of personal interest, calling C-Kira an "Abominable Murderer."

Dejected by Near, C-Kira suddenly stops writing names into the Death Note. On the next page, we see Midora back in the Shinigami Realm, telling Ryuk that C-Kira "let out a squeal," then he committed suicide. This is the last that we see of Midora and C-Kira in Death Note. Ryuk then tells Midora one of Light's statements: the person that is chosen to use a Death Note must have a strong will in order to truly perform divine justice like Kira.