Sisters ~The Invention of Happiness~.As Lala fixes up Nana's De-Dial, Momo tells to Nana grow up while looking at her chest, infuriating Nana. Lala brings out an item that grows Nana's breasts, but it only works for a few seconds and shrinks Momo's breasts. Momo sarcastically admits that she understood Nana's feeling. Later in Mikado-sensei's office, Rito and friends discuss Lala's inventions and Oshizu asks about Lala's childhood. Momo recalls the frequent incidents with Lala's inventions; whenever she was fighting with Nana, Lala's inventions ran wild and they both cleaned up Lala's mess. At lunch, Rito and friends visit Mea's apartment. The apartment is empty and Mea notes that she lives alone. Nana finds it strange knowing that Yami is her sister while Momo was hoping to collect more details about the master. In the follow-up meeting with Mikado-sensei and Oshizu, Momo expresses concerns about Mea and how she may be hiding her true heart. Yami notes that their hearts are incomparable to a human's heart because she and Mea are both weapons. Rito invalidates Yami's claim and relates the sibling analogy to Mikan. The meeting is cut short when a dog clings onto Oshizu, making her rip Yami's clothes, causing Yami to chase after Rito. Afterschool, Ren gives Rito Run's CD and explains about Lala's pure sweet child personality. Rito considers it as he thinks about Momo's harem plan. Later at night, Mikan asks Rito to call Lala and her sisters down for dinner. Rito enters Lala's room and finds her fixing an invention. Rito asks Lala when she started making inventions; Lala replies that it naturally came to her whenever she got bored. Momo hears Rito in Lala's room and thinks Lala is about to kiss Rito but rejects the theory. She and Nana listen on. Rito asks Lala about her incidents; Peke replies that they were incentives to reunite Momo and Nana. Remembering what Ren said, Rito panics as he looks at Lala and activates an invention that wraps around Lala and exposes her privates, and proceeds to tie both of them up. Momo and Nana discuss how much Lala cared for them.