Penguin Brother

Hina's new high school is strange to say the least. She moves back to her old home town only to find that the high school is run by two opposing factors, and students identify themselves based on the color of their school uniforms: Black for the followers of the loud, violent Nishizaki, and white for the followers of quiet, clean-freak Isshiki. And then there are the "grays," who wear street clothes. Hina finds herself stuck between the leaders of the two factions, and it seems to be turning violent.

As one might guess from the girly art and the Ribon publisher, the plot develops a romance element pretty early on in the series. There's a love square involved among the leaders of the groups. Who will Hina choose?

manga.wikia.com

Lamù

An alien race known as the Oni arrive on Earth with the intention of invading the planet. Instead of taking over the planet by force, the Oni give humans a chance to fight for the rights to the planet by competing in a competition. The competition is a variant of the game of tag, in which the human player must touch the horns on the head of the Oni player within one week. The chosen human player is Ataru Moroboshi, a lecherous high school student, and the Oni player is Lum, daughter of the leader of the attempted Invasion.
Despite his initial reluctance to take part in the competition, Ataru becomes extremely interested in the game when he meets Lum. When the competition begins, Lum surprises everyone by flying away and Ataru finds himself unable to catch her. Before the last day of the competition, Ataru's girlfriend Shinobu Miyake encourages Ataru by pledging to marry him if he wins. On the final day of the competition, Ataru wins the game by stealing Lum's bikini top, which prevents her from protecting her horns in favour of protecting her modesty. In celebrating his victory, Ataru expresses his joy at being able to get married; however, Lum misinterprets this as a proposal from Ataru and accepts on live television. Despite the misunderstanding, Lum falls in love with Ataru and moves into his house.
Despite Ataru's lack of interest in Lum and attempts to rekindle his relationship with Shinobu, Lum frequently interferes and eventually Shinobu loses interest in Ataru. Still, Ataru's flirtatious nature persists despite Lum's constant attention. Lum attempts to stop him flirting, which results in Ataru receiving electric shock attacks from Lum as punishment. Lum begins attending the same school as Ataru despite his objections, and Lum develops a fanbase of admirers among the boys of the school, including Shutaro Mendou, the heir to a large corporation. Despite their romantic interest, none of Lum's admirers would risk upsetting Lum by trying to force her and Ataru apart, although this doesn't stop them from trying to get Ataru entrapped from his own behavior.

Wikipedia

Gantz

A pair of high school students, Kei Kurono and Masaru Kato, are hit by a subway train in an attempt to save the life of a homeless drunk who had fallen onto the tracks. Following their deaths, Kurono and Kato find themselves transported to the interior of an unfurnished Tokyo apartment. The pair soon realize others are present and find that they are not able to leave the apartment. At one end of the room there is a featureless black sphere known as "Gantz".
After some time in the room, the Gantz sphere opens up, revealing a bald naked man with a breathing mask and wires attached to his head, and three racks protruding from it, that offer various items for them to use. These include the custom fitting black suits Gantz makes for each of them, giving them super-human abilities, a controller which acts as a radar and stealth unit, and three types of guns.
When the Gantz sphere opens, green text appears on its surface, informing those present that their "lives have ended and now belong" to him. A picture and brief information is shown of some of the Gantz Targets, Gantz ordering them to go and kill them. All but one target shown thus far, have been aliens living on Earth, which take on a wide variety of forms. After a period of time which varies between missions, everyone except Gantz are transported to the location of the mission.
Those sent cannot return from the mission until all enemies have been killed, or the time limit has run out. If they survive a successful mission, each individual is awarded points for the aliens they have killed. They are then allowed to leave, and live their lives as they see fit until Gantz summons them back again for the next mission. The only way to stop having to participate in the missions is to earn one hundred points, and choose the option to be freed. Several participants are killed through the third mission they are given, leaving Kurono as the only survivor and the new leader from the "Gantz Team". However, as the series continues, Kurono participates with the objective to revive his deceased friends with the 100 points he can obtain throughout the missions.
After several missions, Nishi unveils a countdown on Gantz to "Catastrophe" the other players were unaware of. This is thought to indicate the human race will be over in a week for an unknown reason, but it also frees all the participants from the game. A week later, a massive alien force invades the Earth and begins exterminating the human race, while Kurono and his companions try their best to make use of Gantz's advanced technology and weaponry in order to take a stand against the alien invasion.

Wikipedia
gantz.wikia.com