When Mello had kidnapped Takada, Mikami went to retrieve the real Death Note and write Takada's name.
According to his investigations, Stephen Gevanni noticed that Mikami would always go to the bank on the 25th. However, when Takada was kidnapped, he again went to the bank on the 26th. This was unusual behavior for Mikami. It was then Near finally realized about the fake notebook.
Near states that the moment Takada was kidnapped by Mello, Light lost contact with Mikami. As an obligation to Kira, Mikami went to the bank to get the real notebook and write down Takada's name. It was his loyalty that proved to be his undoing.
Gevanni then made copies of Mikami's key card and soon had access to the vault where he was hiding the real Death Note.
Near continues to explain that if what Ryuk said was true, and one could really kill with just pieces of the Death Note, Light must have had a piece available. He wrote down Takada's name, but never thought that Mikami would also do this.
He admits that if things had gone exactly as planned, Light would have won because Near would have never discovered about the fake Death Note. However, thanks to Mello, they discovered the truth.
Then, Lidner states that she realizes now that Mello may have known that this would have happened. She recalls her calling him and telling him of Near's plans, and Mello's only answers was, "Looks, like I'm the only one that can do it." Near, however, disagrees, and does not believe that Mello knew he would die.
Near admits that both he and Mello could never surpass L... individually. But if they worked together... "Together we are as able as L. Together we can surpass L."
And now they have finally avenged L, and Light does not know what he can do now to save himself so Light screams and puts his head down. Then, after some deranged laughter, answers, "That's right. I am Kira."
Ohba said that the chapter title "Answer" was given since Light admits to being Kira even though he previously told himself that he would never admit to being Kira. Ohba explained that Light's answer was like saying "Yeah, I'm Kira. Got a problem with it?" kind of attitude.
"Answer" also refers to Near's explanation about how he discovered Light's true plan and Light's admission that he is Kira. Near's rationale is "very long" in Ohba's words and he understood it is likely that many readers struggled to comprehend it.