Side Effects (副作用(サイドエフェクト), Saido Efekuto ^?) are extensions of human abilities which manifest in people with high amounts of Trion due to the effect of trion on the brain and sensory organs.^[1] There are two types of Side Effect Holders, those who have the ability from birth, and those who later acquired it due to an external factor, however the specific condition to manifest one is unknown, and are rare overall.^[1]

Border has a makeshift ranking system depending on the rarity, but the rank doesn't equal strength, for example Kikuchihara was able to improve Kazama Squad thanks to his enhanced hearing despite being the lowest rank of C.^[2]

• S - Extrasensory Perception (超(ちょう)感(かん)覚(かく), Chō Kankaku ^?), like Jin's Foresight.
• A - Superhuman Ability (超(ちょう)技(ぎ)能(のう), Chō Ginō ^?), like Yōtarō's ability to talk to animals.
• B - Special Constitution (特(とく)殊(しゅ)体(たい)質(しつ), Tokushu Taishitsu ^?), like Kageura's Emotion Perception and Murakami's Enhanced Sleep Learning.
• C - Enhanced Senses (強(きょう)化(か)五(ご)感(かん), Kyōka Gokan ^?), like Kikuchihara's Enhanced Hearing.

Usage

Due to the nature of Side Effects, many holders use their ability in combat, but there are multiple cases where it can be used outside of combat, although there are cases where the holder use their ability either purposely or accidentally without knowing it is a side effect.^[1]

Side Effect Holders

• Yūichi Jin - Foresight (未(み)来(らい)視(し), Mirai-Shi, Lit. Future Sight ^?), which lets him see the future of those before him.^{[citation needed]}
• Yūma Kuga and Yūgo Kuga - Seeing Through Lies (噓(うそ)を見(み)抜(ぬ)く, Uso wo Minuku ^?), which lets him see the lies of others through a visual cue.^[3]
• Chika Amatori: Danger Sensitivity and Enhanced Imperceptibility (敵(てき)感(かん)知(ち)・気(け)配(はい)を消(け)す, Teki Kanchi/Kehai wo Kesu ^?), Chika is the only known person who has 2 side effects - the latter allows her to consciously erase her presence, at the cost of needing to focus heavily, while the former grants her enhanced sensitivity to the presence of Neighbors and Trion soldiers in her vicinity, but not much is known about both side effects.^{[citation needed]}
• Yōtarō Rindō - Animal Communication (動(どう)物(ぶつ)との意(い)思(し)疎(そ)通(つう), Dōbutsu to no Ishisotsū, Lit. Communicating with Animals ^?) which lets him talk to animals, however the intelligence of the animal will determine the response.^[4]
• Shirō Kikuchihara - Enhanced Auditory Perception (強(きょう)化(か)聴(ちょう)覚(かく), Kyōka Chōkaku ^?), which heightens his auditory sensitivity by 5 or 6 times, and increases the precision of his auditory spatial awareness.^{[citation needed]}
• Kō Murakami - Enhanced Sleep Learning (強(きょう)化(か)睡(すい)眠(みん)憶(き)憶(おく), Kyōka Suimin Kioku, Lit. Enhanced Sleep Memory ^?), which enhances stabilization and consolidation of memory during sleep.^{[citation needed]}
• Masato Kageura - Empathic Perception & Synesthesia (感(かん)情(じょう)感(じゅ)信(しん)体(たい)質(しつ), Kanjōjushin Taishitsu, Lit. Emotion Reception Constitution ^?), which lets him perceive the emotions directed at him from people in his vicinity he experiences these emotions as a prickling sensation on his skin. A passive ability that he does not have control over.^{[citation needed]}
• Tsukihiko Amō - Name Unknown (詳(しょう)細(さい)不(ふ)明(めい), Shōsai Fumei, Lit. Details Unknown ^?), but it lets him view others' fighting ability as a visual phenomenon on the light spectrum (coloured aura).^{[citation needed]}
• Yomi - Perfect Parallel Processing (完(かん)全(ぜん)並(へい)列(れつ)同(どう)時(じ)思(し)考(こう), Kanzen Heiretsu Dōji Shikō ^?), which allows him to multi-task at maximum efficiency.^{[citation needed]}
• Sayoko Shiki - Astral Projection (アス(トラ)ル(プロ)ジェ(ク)シ(ョン), Asutoraru Purojekushon ^?), by covering an eye she can project herself to see other things from afar however she can't go through walls or fly in the air.^{[citation needed]}

Trivia

• A recurring theme in the series sees Side Effects contributing to their possessor's misery:
  • Chika's Side Effects allowed her to escape capture at the hands of Trion Soldiers for years, and are thus indirectly responsible for her feelings of guilt after her friend was abducted;
  • Following his father's death, Yūma's Side Effect allowed him to see through the empty platitudes and attempts at manipulation of the people who surrounded him, some of whom would have preferred his father to survive rather than him.
  • Jin's ability to see the future comes with great responsibility, as he may be forced with the choice of letting someone be injured or even die to prevent a worse outcome.
  • Kikuchihara was mocked for the supposed inutility of his Side Effect, with it causing him to overhear himself being mocked.
  • Murakami lost friends due to his Side Effect allowing him to surpass them in their favorite activities in a short time, causing the group to disintegrate.
  • Kageura is not only privy to others' negative feelings towards him, but the prickling sensation provoked by his Side Effect is all the more unpleasant if the emotion directed at him is a negative one.
  • Sayoko made a challenge to prove her power was real. This bad experience would lead her to develop insecurity and difficulty dealing with the opposite sex, especially older men.