Gantz: O

While fighting invading monsters, Kei Kurono rescues his friend, Reika. Though she protests, he faces off against the monsters' leader, dying just after killing it. Reika and her surviving teammates are teleported back to their base. Elsewhere, Masaru Kato dies after attempting to save someone from a knife-wielding man. He wakes up in a small room with Reika and her teammates: Yoshikazu Suzuki, an old man; Joichiro Nishi, a surly teenager; and an angry, unidentified man. Suzuki explains that, in the past, they have each died and woken in the room. They are subsequently forced to fight against waves of monsters. Before Suzuki can explain further, a black orb they identify as Gantz announces their next mission is about to begin. The angry man defiantly refuses to participate, and Nishi kills him, reasoning that he will be a liability.
After putting on skintight black suits, the group is teleported to Osaka; Suzuki, confused, says they are usually sent to Tokyo. When Kato presses for more answers, Nishi threatens to kill him. Suzuki quickly explains more details: the team is given two hours to kill every monster in the city; if they fail, they die. After Kato and his team meet another team, some of whose members have exotic weapons, Suzuki tells him that players accrue points for killing monsters. Anyone who scores 100 points can choose from three bonuses: upgraded weaponry, the ability to resurrect a fallen teammate, or freedom. Nishi turns invisible to stealthily wait for high point monsters. The others watch as the better-equipped team slaughters monsters. Reika and Suzuki urge Kato to stay out of unnecessary combat, but he insists they rescue the city's scared citizens; they refuse, saying they are too weak.
Amused to find Kato fighting monsters alone, Anzu Yamasaki, a member of the other team, follows him, trying to figure out why he is risking his life. Kato says he is a student who lives alone with his younger brother after they were orphaned. Yamasaki reveals she has a young son and says they have something in common: both must stay alive to help someone who depends on them. Although Yamasaki initially believes him to be showing off, she eventually believes he intends to help others and joins him. After killing a monster that was menacing several survivors, Yamasaki proposes that if they survive, the two should live together with her son and his brother. Although surprised, Kato agrees, and they rejoin the others.
A gigantic monster rises from the water, and an equally large mecha appears, piloted by seven-time winner Hachiro Oka. Oka kills the monster but loses his mecha. Two other multiple winners die fighting, when they become overconfident. Oka, wounded and fighting without his best weapons, engages the leader, who appears first as an old man, a naked woman, and finally a skull-headed demon. Oka appears to win but tells them it is not dead; instead, it must be taken by surprise to be killed, and his current weapons make it too dangerous. After rising, the demon easily defeats Suzuki, Reika, Yamasaki, Nishi, and Kato. Suzuki is heavily wounded when he sacrifices himself to save Kato, and Nishi's arm is ripped off. Instead of killing them, though, it goes after Oka, who has left to retrieve better weapons.
Knowing someone must defeat the demon soon and Oka may be too wounded, Kato acts as a decoy in a trap. The demon, having killed Oka, attacks Kato, and the others shoot it from a distance. Before dying, the demon kills Yamasaki and maims Kato. Kato collapses beside Yamasaki's body, and the others assume him dead. Reika, Suzuki, and Nishi are surprised when he rejoins them at the base, where all are fully healed. When the scores are tallied up, Kato has 100 points; Reika and Suzuki encourage him to choose his freedom, but he resurrects Yamasaki. After Gantz allows them to leave, Kato races to return to his brother. The others discuss how Kato previously won the game, had his memory wiped with his freedom, and has somehow returned, unchanged from his previous willingness to sacrifice himself for others.

Characters

Masaru Kato

Masaru Katou has a good personality; he loves to help people and easily cares for everyone. His parents are dead so he is responsible for his younger brother who he is fiercely protective of. Katou usually takes the position of being a leader because he has good authority, but is unable to hurt others which hinders his performance in the game of Gantz.

Masaru Katou was Kei Kurono's childhood friend. They meet each other again the day they die because Katou was trying to help a drunk off of the train line, with Kurono reluctantly lending him a hand.

Anzu Yamasaki

Part of the Osaka Gantz Team.

She first sees Kato Masaru help a family from one of the aliens. Bewildered by his actions, she asks him what he was doing. When he responded that he was saving the family, she immediately laughs at him. Initially she follows him around while waiting for him to show a contradiction in his actions and words.

When he does not show one and even puts himself in danger to save others, she leaves his side. Feeling strangely guilty, she returns to help Kato defeat the alien he was facing. Unfortunately she gets in his way more than anything else and Kato ends up rescuing her. Yamasaki continues to follow Kato throughout the next few chapters, and finds herself attracted to his kindness and compassion. In the end she tells him about her son (she feels closer to Kato after he tells her of a similar situation between himself and his younger brother).

After Kato announces that he will act as bait for the 100 point alien, Yamasaki asks Kato if he and his brother will move in with her and her son. In order to make this reality, she vows to not let Kato die no matter what.

Reika Shimohira

She is a beautiful, young, and popular idol. Like almost everyone else, she is caught in the line of fire during Izumi's killing spree in Shinjuku. She develops a crush on Kurono after seeing him in action during her first night as a participant in the game. Surprisingly enough, she proves to be a formidable opponent against the Aliens.

Joichirou Nishi

Jouichirou Nishi is a middle-school veteran of the Gantz world, having arrived there over a year before the beginning of the series. While running away from police officers after shoplifting, he fled onto an apartment building roof where he fell to his death.

Much like how Kurono started out, he is also selfish and has a Social Darwinist approach to life. Nishi is reluctant to reveal anything about the Gantz or himself, but he runs a web site where he posts information about it (carefully avoiding Gantz's rule about informing outsiders, which would in turn prove fatal). Gantz calls him "Mr. Nishi," which is rather interesting given how informally or derisively he refers to most of the other participants.

Preview
Trailer 1

Trailer 2

Reference:
https://en.wikipedia.org
https://myanimelist.net
http://www.1999.co.jp