A boy seemingly unknowing of the ways of the world—including the concept of blood—who nonetheless braves it in order to search for his former caretaker, Karoku, an enigmatic figure who parted ways with him before the story began.
Everything's Better with Rainbows: Its what his species name, Niji, means. He originally lived in the Niji Forest, and he later somehow casts a large rainbow in order to find Karoku.
Good Is Dumb: Justified, considering that he was originally a cute little animal, and looks like and is later referred to being a child as well. However, unlike some character types who remain clueless throughout the story, he's slowly learning more about the world.
Red Eyes, Take Warning: Subverted, he is incredibly naive and would barely hurt a fly unless an animal was about to kill Karoku.
Super Senses: Despite his not having any offensive abilities at the moment, Nai is far from being The Load, as his elevated hearing has more than once been crucial in ensuring the survival of those around him, and later, useful during missions.
An intelligent and resourceful young man, Gareki starts the story engaged in corrupt business before things take a very wrong turn. In the midst of the chaos that ensues, he somehow saves Nai from the vile woman he originally meant to rob, and becomes involved in the mystery surrounding the boy while they are on the run.After Circus takes Nai into their custody, he's determined to stay for his own gains, but in truth he may care more about those around him than he likes to let on, even to himself...
Defrosting the Ice Lord: Nai (unwittingly) and Yogi (determined) together with other Circus members have gotten him to open up considerably.
Gadgeteer Genius: Made particularly apparent through his contribution to the mission to save Karoku in volume 7.
Goggles Do Nothing: Averted. He's wearing them in his first appearance, having just blown up a portion of the mansion where he meets Nai. They don't get much use after that, though this is justified by the fact that he has no need for explosives after being taken aboard Circus's second ship.
Handguns: Gareki likes using them. Circus doesn't like his use of them.
Jade-Colored Glasses: The losses in his childhood, which includes being sold as a slave by his parents and seeing his Parental Substitute being exploited, as well as life on the street have made him very cynical about people (himself included) and unwilling to make friends.
Street Urchin: Gritty type. Prominent early on, though the circumstances over why are explained later. He does mellow down once he and Nai wind up with Circus, becoming broodier and using less dirty tricks. His sharp intellect's evident throughout, however.
The Smart Guy: Despite the implication of his street savvy, Gareki's shown and later noted to read frequently during his time in Circus, which heightens the poignancy when what he was reading up on is revealed in a conversation with Hirato in chapter 38. His skills as an unsung Gadgeteer Genius backs this position up further.
Stuff Blowing Up: Is very knowledgeable about explosives. He was even introduced setting up a bomb.
The Gunslinger: What he might be, if Circus allowed him to use his gun.
Tsundere: Gareki can be a Type A male version, especially when Yogi teases him.
In fact, both Yogi and Tsukumo are awestruck at his surprising innocence regarding snow (which he had never seen) at one point in the manga.
The mysterious young man whom Nai is looking for. Little is known about him, but it appears that he was always with Nai and suddenly disappeared one day, leaving behind a trail of blood and an outdated Circus bracelet. It's revealed that there are actually two Karokus, and chapter 57 confirms them to be brothers.
Evil Twin: It's very possible that the Karoku who is working for Kafka is this to the one who took care of Nai.
Kidnapped Scientist: If he really was involved in the creation of Nai, who's successfully existing as a new hybrid species, then it is very likely that he was abducted for his knowledge of genetics as well as his ability to create a stable life form, an aspect in which Kafka's valga have fallen short.
Nice Guy: Nai thinks of him as "as warm as the sun". Later events hint that there is something sinister about him
Turns out he was a nice guy, its just that there were two Karoku's.
Not So Stoic: In chapter 52, after regaining his memories of Nai, he loses his usually composed disposition and breaks down in tears when Nai is close to dying from his injuries.
Psychic Link: He seems to be able to communicate with Nai this way under certain circumstances.
Teen Genius: Heavily implied to have had a hand in the creation of Nai as a Niji-human hybrid.
Trauma-Induced Amnesia: After he's rescued from Kafka by Circus, his memories become so scrambled that he entirely forgets his relationship with Nai. Dr. Akari suggests it's because of the stress of his previous circumstances.
Magnificent Bastard: exhibits shades of this, as he always seems to have a plan and is one step ahead of everyone else.
Noble Demon: Says to Akari rather wistfully that it's his job to take lives but as seen in flashbacks with his first encounter with Yogi, Hirato is not quite as ruthless as he'd like everyone (and himself) to believe. Not to mention how protective he his of his "children".
Ymmv, but he also has shades of Team Dad and A Father to His Men. His affection for his crew is more concealed than most examples of thiese tropes, and may manifest in strange ways (due to his love of provoking others), but he does things like buying candy and souvenirs for Nai and arranging for Gareki to attend school in the hopes of getting his license to officially join Circus. These tendencies are perhaps best viewed in his interactions with Tsukumo, though ymmv on whether you read those in a romantic context or not.
Tranquil Fury: Though almost impossible to read most the time, he has a moment of this as early as chapter 14 when, after Yotaka fought against and injured Yogi, he pursued the person responsible and just after talking with him calmly, says, "It is enjoyable to torment, isn't it?", (even sporting a pleasant smile to boot) before having his banshees tear him apart.
Another time this happens is in chapter 35 when Azana betrays and tries to kill Akari. He doesn't smile this time or have any sarcastic remarks, but he knocks Azana away and calmly states the punishment for attacking someone of SSS rank before attacking. If Akari hadn't stopped him, he would have killed Azana without a second thought or any emotion on his face at all despite his anger. http://mangafox.me/manga/karneval/v06/c035/7.html Do not dare to hurt akari.
Green Thumb - Yogi's weapons are two thorn-like épées, and he can conjure up thorny vines with them as well. Combined with Scarf of Asskicking in chapter 19, whereupon the Badass split personality of Yogi saves Gareki from the split personality's own attack this way.
Hidden Depths: Yogi can be melodramatic and flamboyant one moment, then more serious than the First Ship's female fighter the next. There's definitely a lot of this going on here.
Hidden Depths: While he comes off as much more laid-back than Hirato and Akari, his expression turns quite sinister at one point when he asks Tsubame what she's willing to sacrifice to protect the people around her.
Foil: The noisy, cheerful and seemingly silly Captain to Hirato's calculated and suave manners. He's by no means incapable though.
Friendly Address Privileges: Loves to call Akari-sensei "Akari-chan", a fact that irritates the doctor as he is older than him. According to the short extra after chapter 72, he even called him that when akari used to be his instructor in Kuronomei.
Bitch in Sheep's Clothing: Zig-zag. Underneath his politeness, he is actually aloof and capable of nasty words and actions, as Gareki learns firsthand. On the other hand, he's dedicated to his comrades, as a true Circus member will.
Dye Hard: His hair is actually gray, but he constantly dyes it.
Bunnies for cuteness: The flower bunny that he takes care off is meant to represent his soft side. In the fanbook it is stated that it bring him joy to take care of it. I mean the bunny refuse to return to the wild to stay with him. You cannot get any cuter than that.
Not So Stoic: When he discovers that Azana was poaching in vinto and then when he tells him that he hates him. It is far more dramatic in the CD drama, but the face he makes in the anime was really sad. ◊
OmnidisciplinaryScientist /SuperDoc: Not only does he have medical knowledge, he also has knowledge and does research concerning biology, archeology, genetics and pharmacology as he is the Research Tower´s Research Chief Specialist. Relating to his medical knowledge he is also a surgeon, an expert in emergency medicine, fixing fractures, making his own medicine (Yogi's allergy patch). That indeed takes a lot of different medical specialties. No wonder he is a SSS rank.
Vitriolic Best buds: Although he acts antagonistic towards Hirato and Tsukitachi he ends up spending his free time with them drinking (initially not willingly of course). Both Hirato and Tsukitachi both care for him as well and make sure he takes a break once in a while. They were both worried for him after the incident with Azana.