Answer

The Answer is the sixth chapter of Volume twelve and 104th chapter overall.

Plot
Near shows Light an interesting name written on the real Death Note: "Kiyomi Takada, commits suicide, January 26, 2: 22 p.m."

When Mello had kidnapped Takada, Mikami went to retrieve the real Death Note and write Takada's name.

According to his investigations, 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, Linder 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 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! What is Light to do?

Light screams and puts his head down. Then, after some deranged laughter, answers, "That's right. I am Kira."