Weapon

 Weapon is the 4th chapter of Volume four and 29th chapter overall.

Plot
Misa is amazed that Kira is so young and good-looking. She looks up his address.

On May 25th, the Kira investigation task force is meeting with L and Light. They note that they discovered no unusual behavior on the 22nd in Aoyama and the 24th in Shibuya. Light wonders if he should go to the Tokyo Dome like the diary says to meet the second Kira since he obviously didn’t see her in Aoyama.

Suddenly, Watari sends the group a message sent by the second Kira that was postmarked on the 23rd. The second Kira says in the video that he has found Kira. Light is confused because he didn’t see anyone with a notebook at Aoyama. L has deduced that the second Kira found Kira on the 22nd in Aoyama, when Light was there. L orders the police to air a message offering immunity for the second Kira in exchange for information on the real Kira. Shocked at hearing this message, Misa decides to visit the real Kira.

Misa introduces herself to Light as Misa Amane and then lets him touch her notebook. Light sees the shinigami Rem. Light has finally found the second Kira.

Light leads her up to her room, telling her mother that she’s his girlfriend. Then, Light asks her how she knew he was Kira. Misa explains to him about the fact that people with shinigami eyes can’t see another Death Note owner’s lifespan. Misa offers to be Light’s “eyes” if he can make her his girlfriend. Misa tells Light that she has been careful covering her tracks using disguises and letting other people put their fingertips all over the packages in which she sent the tapes. Light is being cautious because she has the power to kill him. However, Misa offers to give Light her notebook so that he can make sure she won’t kill him, and she can gain his trust. Misa tells him that she doesn’t mind if he uses her and kills her if she becomes a burden. She tells Light that Kira is like a god to her because he killed the criminals that killed her parents. She will do anything for Light. Light agrees to let her help him because he could use her as a valuable weapon… to kill L.

Afterwards, though, he plans to dispose of her.

Conception
The title "Weapon" refers to Light's idea of using Misa as a "weapon" to Kill L. Misa even still accepts Light's attitude towards her.