Humanoid Abomination: It is a hairless comatose man wired inside of an indestructible black ball that carries weapons, power suits, teleportation capabilities, and poor instructions on finding and killing deadly alien invaders.
Jerkass: Deliberately spites, insults, and degrades the players, even when they are at their lowest.
Killed Off for Real: You know shit has gotten real when the Giant Aliens attack the Gantz room, blowing the black ball out of the building and into the street, crushing the head of the man inside. Though, there's more where that one came from.
Troll: Like no other. Disrespects and inconveniences the players at every turn, delays the firing mechanism on all of the guns, alters the rules of the game on a whim, and targets aliens that are either a) worth next to no points, or b) the least dangerous out of several that are extremely dangerous. Worst of all, it doesn't do it because of programming or anything, it just likes fucking with the poor souls it traps.
Unwitting Pawn: As much a slave to forces beyond its control as the players it plagues. Although its powers are still near godlike and can be used to deadly effect no matter whose hand is controlling it.
We Have Reserves: Not just the endless supply of potential Hunters, but there are literally thousands of Gantz balls in production, waiting to make life hell for humans and aliens by the score.
Well-Intentioned Extremist: While it can't be denied that many of the creatures Gantz targets need to be stopped, the methods it uses to achieve these goals are cruel and unusual.
Character Development: Goes from a man who's only really concerned about himself and sex. To a Badass, risking life and limb for someone he loves.
Designated Hero: Starts off as one, a selfish and mean-spirited teenager obsessed with sex and not much else. His experiences with Gantz (as well as postive influence from Kato) set him on the path to becoming a more mature, heroic person.
The Determinator: He's horribly outclassed, but his will to survive is strong enough that it doesn't matter.
The Unfavorite: His parents could give a rat's ass about him, preferring his outgoing younger brother Akira. His teachers don't respect him. And the only friends he has at school are disloyal and superficial.
World's Strongest Man: Can sometimes give off this impression. Since he is essentially the one who defeated The Giants and championed humanity, he may truly be this at the end.
Beware the Nice Ones: Despite his large stature and troubled background, Kato is probably the most sensitive and caring male character in this series. In spite of his large stature and troubled background, he is a force to be reckoned with when forced to defend himself or others.
Good Is Not Dumb: He is about as morally upstanding as they come, but he'll still pre-emptively ambush a guy on the toilet in order to defend himself and head off further violence.
Guile Hero: While he at first comes off as a delinquent turned Wide-Eyed Idealist, Kato is shown to be a particularly good liar when he has to be and is even skilled at manipulating fights to his advantage.
Reluctant Warrior: Several of the Gantz team members (most of whom didn't exactly volunteer) aren't real gung-ho about the idea of hunting and killing aliens, but Kato is probably the purest example. Most notable in the initial hunt, where he tries to stop the others from killing the first Onion Alien and then tries to talk to the second one (after watching it slaughter several people). Later on he gets more accustomed to his teammates' use of lethal force to the point where it doesn't make him suffer a Heroic Blue Screen of Death and break down crying every time, though he still personally tends to almost always at least try to use the capture/Teleport Gun whenever possible as opposed to one of the "ludicrous gibs" type, and risking himself to save others is one of his defining character traits.
What Happened to the Mouse?: The Kishimoto who Kurono and Kato loved and lost was a clone. The real Kishimoto survived her suicide attempt and later, the alien invasion, living to the end of the series.
"Remember how I said you had a little bit of potential last time? I was wrong. You should be the first one to die!"
Eye Scream: Nishi, when he is killed by Suzuki Seiji
Freudian Excuse: It's never explicitly stated, but considering he was thrown out of several-storey window by bullies he refused to back down against in an attempt to kill him. (Notably, when the bullies try to cover things up ""his entire class goes along with it.""), it's fairly freaking unlikely that his life is rosy, by any definitions of the term.
Also, near the end of the manga, Nishi acts like everyone is insane when they decide to use the teleportation ability of the Gantz spheres to rescue captured humans on the enemy mothership rather than immediately striking its weak point to destroy it. He's right, and humanity almost loses because of the delay.
Killed Off for Real: He bites the dust in Chapter 366. It's interesting to note that Nishi, of all people, cries out for his mother as the alien ship disintegrates.
Tsundere: In the Gantz PS2 game if you follow his route. He even says something along the lines of "Don't get the wrong idea; I didn't come here to help you" during one event. This is mind-meltingly weird for anyone who's familiar with the Jerkass Nishi from the manga and anime, to say the least.
Villainous Breakdown: Basically every time he finds himself at a disadvantage, he starts losing his shit and whining for the people he ruthlessly antagonized earlier to help or spare him.
Younger Than They Look: He looks like he's 18 at the youngest. He's in middle school. Most likely done to save the audience the horror of seeing someone barely into puberty committing the atrocities he does.
Morality Pet: Hoi Hoi is his literal morality pet. Also his girlfriend, who he was at first indifferent toward but ended up dying for. He even tells her he's going to take her to Disneyland in his last moments.
Nominal Importance: Subverted. When he's first introduced, we don't learn his name for ages, and as an unassuming middle-aged guy with an aversion to violence and no immediately distinguishing character traits beyond general geniality, he seems like he's certain to die without lasting one fight... but he becomes one of the main cast and survives longer than almost anyone.
Knight of Cerebus: It is quite a devastating display when he decides to really cut loose on an enemy. The rest of his team would agree.
Made of Iron: The first time he dies, Kaze is riddled with bullets at point blank range by Izumi while he himself is crushing Izumi's throat. He doesn't go down until the gunfire cuts his legs apart, leaving him crippled and vulnerable.
A Man Is Not a Virgin: Subverted. He's one of the most competent fighters in the series...he's still a virgin though.
Manly Tears: After finding out Takeshi's 'stepfather' beat him to death over a pudding cup, while his mother did nothing to stop it.
The Fatalist: After a few missions, Sakata, tired of the life and death situations Gantz puts he and others through, tells his teammates not to resurrect him if he dies. These are even his last thoughts, which he conveys to Sakurai telepathically.
Fish out of Water: The Osaka Arc is his first time as a Gantz player. The only reason he survives is because he and his friend were smart enough to wear the suits, and because more than a few people take the time to save him.
Irony: Of all the Osaka members, Anzu lives possibly the most morally ambiguous normal life. While most of the Psycho for Hire types you read about earlier are shown to be mild mannered members of the workforce in their normal lives, the sweet and attractive little Camper Anzu is shown to be passively neglectful toward her son so she can focus on earning money. By illustrating porn. In her living room.
Morality Pet: Love for her son compels her to survive. Love for Kato compels her to be heroic.
The Oathbreaker: Dies to prevent Kato's death, even though she made him swear earlier not to die so that they and their families could all live together. They each tragically lampshade this as she is being killed.
Combat Pragmatist: Uses the most powerful weapons and armor Gantz has to offer not to protect the weak innocent, but to increase his odds in battle despite the fact that he is plenty dangerous without all of it.
Cruel and Unusual Death: Apparently ripped apart by Nurarihyon. We only see that he's missing everything below his chest.
Curb-Stomp Battle: He's cleared seven missions with 100 points. At the time, it was considered next to impossible to get more than a few points each mission.
Know When to Fold 'Em: It turns out that the secret to surviving a long time in Gantz is to back off when an alien is obviously too powerful for you, and just run out the clock or hope someone else kills it. Who knew? (It doesn't save him this time, because the alien is a Blood Knight who wants to keep fighting him, but it's almost certainly how he survived so long in the first place.)
Lack of Empathy: Does not feel the least bit bad about leaving the scared and uncertain Tokyo Team to deal with the monster even he can't kill.
Straight Edge Evil: In the Osaka spinoffs, Oka appears to be a well-adjusted office worker by day, quiet and efficient. Though, he is clearly bored, and even more clearly exhilarated when he receives the signal for his next and last mission.
The Worf Effect: It appears that Oka (who has already taken down a Godzilla level monster) can be the only one capable of defeating the Nurarihyon Alien that has so far eliminated three of his best teammates, dominated half of the Tokyo Team, and proven itself unkillable. Even with all his wonderful toys, this turns out to not be the case. Since both got a lot of buildup, this avertings it for the monster, but plays it straight for Oka himself.
Worthy Opponent: Nurarihyon considers him to be something like this, claiming "he was fun" when confronted with Kaze's "predictable" martial arts. It even abandons its fight with the Tokyo Team to hunt Oka throughout Osaka, and destroys several buildings in the process of killing him.
Cruel and Unusual Death: Suffers one of the cruelest and most unusual deaths when he is hit by one of the Pandora's Box spores, turning him into a violent mutating mass of his own extremities, clothing, and weapons.
Dying Moment of Awesome: Killed immediately after delivering some serious blows to the nigh invincible God of War Giant, including cutting off some of his fingers, and letting everyone know that they can beat him.
Benevolent Alien Invasion: The Onion Alien and his father don't seem to be hurting or planning to hurt anyone until the Gantz Hunters find them. Though, the father is capable of serious damage if provoked.
Shoot the Dog: The son is hunted down and shot to pieces by a group of ignorant and/or violent new Gantzers. Just when it looks like the father has turned the tables on all of them, he is taken down by Nishi.
Innocent Aliens: Allegedly. They pay for food they take from the store while invisible and they don't attack anyone until they are provoked. However, accidentally stepping on one of their baby chick siblings is usually enough to provoke them. And if so, there are likely many innocent bystanders who stepped on them while they were invisible and got killed as a result.
Bling of War: All of their statue disguises can count as this, especially 1000 Arms.
Blood Knight: Innocent or not, these aliens are talented, destructive fighters. When it can speak english, 1000 Arms is almost psychotic in its enthusiasm about beating Kato to a pulp.
Break the Haughty: What this mission symbolizes for Kurono. His over-inflated ego is quickly demolished when the girl he loves is melted in front of him, he loses An Arm and a Leg, and finally he finds out that he is the mission's Sole Survivor.
Innocent Aliens: 1000 Arms claims they never did anything to hurt humans and have no idea why they are being attacked. Since they are specifically in disguise as peaceful statues, it probably was not lying.
Knight of Cerebus: Played with. The Rowdy and Grumpy Aliens seem like this compared to the aliens in the first two hunts. Then the towering (and by far the largest) Buddhist statue breaks out of the temple... and is unceremoniously killed by Kurono. Once these titans have been taken down, it looks like easy street for the team. Then they find The 1000 Arms Alien.
Pyrrhic Victory: Kato fights to the bitter end, finally slaying 1000 Arms and rejoicing that he will get to go home to his brother. Then he's killed by the alien's final attack, seconds before he would have been sent back.
Time Master / Healing Factor: Another of its statues ornaments can reverse any damage that it takes, so long as the ornament survives.
Symbiotic Possession: Consumes a nerdy Gantzer's brains and adopts his personality to communicate with humans. The guy realizes this, but then he begins to like how powerful he feels in the alien's body.
Boldly Coming: A truly horrific example when Inaba and Reika start having sex in the middle of the blood soaked mall. The real Reika shows up and the one on top of Inaba begins transforming and attacking the others while he is still inside of it.
World's Strongest Man: Or aliens. Hundreds, possibly thousands of Gantz teams from all over the world are sent to fight these guys. Those hundreds of thousands end up barely having any time to breathe, let alone fight back.
The Ace: The God of War appears to be this among the Giant Alien Army...
Broken Ace: But is probably this after the Giants have been defeated and his brother has been killed.
Crazy-Prepared: They come totally ready to cripple the secret countermeasures of Gantz, as well as military and governments, and devise several devious ways of putting the Hunters through hell. Not that it helps them win in the end.
Body Horror: Some of these aliens can explode into a cloud of floating light spores. When the light spores touch humans, they enter their bodies and mutate them from the inside out, appendages multiplying like tumors, turning the victims into violent freaks of nature.
Knight of Cerebus: The largest of these aliens is some sort of deformed giant with a scowling human face and mysterious abilities. It's so terrifying that the whole team finally decides to run, save Kurono's Clone who wants to prove himself by killing it, and Reika who doesn't want to leave him. After the creature beats Kurono into submission, it fights Reika, and the two kill each other.
Mighty Glacier: the largest and most powerful of these aliens hovers over the battle and watches mostly. After it drops to the ground, it's movements are very ponderous. However, its telekinetic powers and ability to summon an entire new brood of creepy-crawly aliens makes up for it.
Expy: They seem to be a freaky combination of the aliens from Contact, the Engineers from Prometheus, and the monolith from 2001: A Space Odyssey.
Good Is Not Nice: Technically, they gave humanity the means to defend itself from dangerous alien invaders. However, the information was given to the mentally challenged daughter of a ruthless German businessman, who used the Gantz technology the way seen throughout the series: enslaving resurrected humans to a sadistic artificial intelligence and pitting them against bizarre, horrifying, highly lethal creatures. The Room of Truth Aliens did not do any of this out of the kindness of their hearts, nor out of any other great concern of theirs. More likely, they just want to maintain some sort of balance in the universe, which was likely being screwed up by the world destroying Giant Aliens.
They more or less state that they did what they did in retaliation for the Giant Aliens trying the same crap on them. Basically, "They pissed us off, so Let's You and Him Fight."
Evil Is Petty: Acts this way, but not quite. It seems like killing The Hunters started out as business to him, then it became personal, but after he ends up a member of the Tokyo Team and witnesses what they have to go through from their perspective he loses interest in wiping them out.