Chapter 6 Summary
So Rina remembered about the whole affair where she shot her mom and dad as they tried to kill the brother. She lost her memories and identity as she couldn't bear the shock after that. As seen in the manga, Rina tried to kill Misaki to protect her brother, but the brother broke out of the paralysis in time to stop Rina. The neo-Rina cried as she realized that she's about to disappear since the old Rina is resurfacing. She asked the brother who she really is, and the he replied that she will always be the love of his life. The new Rina heard that, smiled and told the brother not to forget about her even if she will be gone. The brother took Rina back home and told Misaki to come with him if she wants to meet the real Rina.
It turns out that whenever Rina regains herself she will always relive the scene where she shot her parents and will panic as she does not recognize anyone around her. This is why the brother was so adamant to protect the new Rina as he has no way to pacify the old Rina. Misaki and the brother lost their hopes of bringing Rina back and started to have a sex-a-thone to console themselves. After a few days of that cycle, the original Rina seemed to regain her self-control and started talking with Misaki about stuffs they did when they were kids, only to lapse back into her panic the next day. In their despair, the brother took out the pills the fortune teller gave him back in chapter 4 and decided to drink it with Rina. He thought there were 2 pills, but it turned out there were 3 pills in total, which means Misaki is included as well. Misaki seemed to know who the fortune teller is, and they resolved to commit suicide via the pills since they thought those are poison.
All three drank the pills, but when Misaki woke up nothing seemed to have changed. However, Rina appeared once again with a phone call from the fortune teller saying that she's taking care of Misaki's cats right now. Rina seemed to be familiar with both cats, which let Misaki knows that the two Rina had merged together (each Rina only knew about one cat and not the other.)
The real shocker this time is that the brother woke up without any memories, and Rina told him that they are siblings and left the town together. Misaki's dad died shortly after, and the other gangs took over her dad's territory, so Misaki left the town and started a new normal life. In the ending, the one ringing the bell is probably Rina since the cats recognized their former owner and started meowing.
PS: A little subplot here: the girl that was forced to have sex with the brother in chapter 4 committed suicide with her family after that short stint due to their family's debt. The brother speculated that Misaki had her goons checked their family out and called for the ambulance since the family was discovered very soon after their group suicide. A newspaper article in chapter 6 revealed that the family had regained consciousness, and the brother informed Misaki about this. She remarked that it might have been better to let them die since the girl's family still have to pay back the debts if they live.