Chika Amatori's Side Effect

Short Summary
''After Osamu is threatened with expulsion from Border, Jin meets him, and is introduced to Yūma, discovering Yūma is a Neighbor. Yūma then reveals that the irregular gates were caused by a Neighbor, and Border makes an effort to find and destroy every last one of them. By Yūma's request, Osamu is given the credit for the information. Later, while waiting for Osamu, Yūma meets Chika Amatori, a mysterious girl who seems capable of feeling the presence of Neighbors. When a danger alarms sounds, Chika runs to the Forbidden Zone, and Yūma follows her, saving her from a Neighbor. Osamu, who became a B-Rank Agent, defeats the Neighbor. Yūma then learns Chika's secret, and that she and Osamu already know each other, to his surprise.''

Long Summary
A young Amatori Chika and her friend pause on the way home from school. Her friend reassures Chika that she believes in and will protect Chika from the "mysterious things" in the world as a set of eyes watches them from the bushes. As Chika is about to enter her house, she senses danger and rushes to the street and finds her friend's bag lying on the ground.

Back to the present day, Osamu lies in bed late at night contemplating the day's events: the Trion barrier set up by Border can only seal off the irregular gates for two days, and although Jin Yūichi has no idea what to do, he claims Side Effect tells him it will be okay. Osamu discusses Side Effects with Replica and Yūma via Replica's clone while Yūma rummages through the wreckage at school for information on the cause of the irregular gates.

The next morning as Osamu leaves his house, he is unaware of Shūji Miwa and Yoneya Yōsuke watching him as they discuss the humanoid Neighbor. They are surprised when Jin suddenly appears behind them and shows them an order to report to Headquarters. Jin then meets up with Osamu and explains they are going to meet someone who knows about the irregular gates, someone Osamu knows. It turns about to be Yūma, who is standing in the place where Yūma defeated the Bamster a few days ago. After brief introductions, Jin casually asks Yūma if he is from the other world. Osamu and Yūma are shocked, but Jin quickly explains that his Side Effect allows him to see the future. The day before he saw an image of Osamu and meeting someone with Osamu who would tell him about the irregular gates. Yūma reveals a damaged Rad modified with a device for generating gates. Replica explains these Rads were hidden in the belly of the Bamster Yūma fought. They hid underground after separating from the Bamster and spread throughout Mikado City, using the Trion of people nearby to open the gates. Osamu is depressed to learn there are thousands hidden throughout the city, but Jin thanks them for the help and state it is now a job for Border.

At Headquarters, Jin (the self-proclaimed "Power elite") quickly gives directions to the members of the Administration on how to handle the Rads. Under his command, Border agents quickly find and destroy all of the Rads, to Yūma surprise. Jin expresses his gratitude to Yūma and Replica, wishing they were in Border so they could receive recognition. Yūma suggests the credit should go to Osamu. This move is supported by Jin, who states that this means Osamu will be promoted to B-rank as an official agent and could then fulfill his dream of saving someone.

Later, as Chika waits by the river, Yūma crashes nearby. He tells her he is waiting for someone as well and is practicing riding his bike, a feat which mystifies him. Chika receives a phone call and then decides to help Yūma learn to ride his bike while she waits. Yūma finally gets the hang of it, but crashes into the river. After pulling himself and the bicycle out of the river, Yūma introduces himself. Chika suddenly senses something and turns around just as an alarm sounds in the distance. Yūma reassures Chika, saying it is off in the Forbidden Zone. Remembering her missing friend, Chika runs off in the direction of the alarm.

Chika enters the Forbidden Zone and hopes that since she is there, the Vander will not enter the city and attempts to stay hidden. Her phone rings; however, alerting the Vander to her presence. Just as it strikes, Yūma jumps in and saves Chika. Replica advises against using his Trigger, since Border has begun a fight nearby. He adds that Osamu is on his way, and Chika reacts to the mention of his name. A little way off, Osamu sees the Vander and activates his Trigger, transforming into his B-Rank form. Worried he won't be able to defeat the Vander, Osamu reminds himself he is now a B-Rank agent and manages to fire a shot damaging the Vander's eye. Although he has difficulty controlling his thruster, Osamu is able to destroy the Vander with one hit. He is congratulated by Yūma, who appears with Chika. Osamu demands to know why she entered the Forbidden Zone, and Yūma is surprised to find out they know each other. Osamu explains he wanted the two of them to meet today. He then asks for Yūma and Replica's insight, because Chika is a human who attracts Neighbors.

Characters in order of appearance

 * Osamu Mikumo
 * Yūichi Jin
 * Replica
 * Yūma Kuga
 * Yōsuke Yoneya
 * Shūji Miwa
 * Ai Kitora (Flashback then Present)
 * Motokichi Kinuta
 * Eizō Netsuki
 * Masafumi Shinoda
 * Miscellaneous B-Rank and C-Rank agents
 * Jun Arashiyama
 * Mitsuru Tokieda
 * Chika Amatori (Debut)
 * Aoba Harukawa (Flashback, Debut)

Triggers in order of appearance

 * Kogetsu (Miscellaneous Agents)
 * Asteroid (Miscellaneous Agents, Osamu Mikumo)
 * Scorpion (Ai Kitora)
 * Raygust (Osamu Mikumo)
 * Thruster (Osamu Mikumo)

Adapted Chapters

 * Chapter 11
 * Chapter 12

Differences Between the Anime and Manga

 * The scene between Osamu and Jin discussing the irregular gates doesn't happen on a rooftop in the manga, instead appearing to happen inside of a building.
 * The "No Trespassing" sign is shown twice in the anime, as to only once in the manga
 * Osamu saying it's impossible to exterminate all the rads is an anime only line
 * Arashiyama Squad only has a panel dedicated to them in the manga, as in the anime they get a short segment of being live on the news
 * Chika's flashback with Aoba is not present in the manga