Sainan Metropolitan High School or simply Sainan High (彩南高校) is the school where all of the main characters (except for Mikan) study. As the series progressed, more and more of Rito and Lala's friends enroll in the school, usually girls. A running gag in the series is that the female characters in the series do not have any difficulty enrolling despite being aliens because the perverted principal accepts any cute girl who enrolls there. Sometimes, the principal would even invite other cute girls to go to his school (i.e. Riko). However, there were times where a male character was able to enroll in the school easily as well, like Ren, who enrolled in the middle of the year. A reason for this could be that the principal's pervertedness does not pick any gender (like in one case when the principal chased after Run, so she transformed into Ren, hoping that the principal would stop, only to have him say he was okay with that too, and resume chasing).
During summer, the male unifom consists a white short sleeve button shirt and green trousers with a yellow cross-hatched pattern. In Winter, they wear a lemon coloured blazer and a green tie.
During summer, the female uniform also consists a white button shirt with a sleeve-less lemon colored jumper, a green bow-tie and green skirts with yellow coloured cross-hatched pattern. In Winter, they also wear a lemon colored blazer instead of a jumper.
This list only includes characters who have names.
- Momo Belia Deviluke
- Nana Astar Deviluke
- Kurosaki Mea
- Yami (Golden Darkness)
- Yūki Rito
- Lala Satalin Deviluke
- Sairenji Haruna
- Kotegawa Yui
- Ren Elsie Jewelria
- Run Elsie Jewelria
- Sawada Mio
- Momioka Risa
- Saruyama Kenichi
- Oshizu Murasame
- Arai Sayaka
- Shirayuri Koyomi
- Matome Ageru
The first ones to be introduced as students of Sainan High were Rito Yuuki, his male friend Kenichi Saruyama, and Rito's crush Haruna Sairenji. In the second chapter, Haruna's perverted friends, Risa Momioka and Mio Sawada, were also introduced briefly. Then, by the fourth chapter, Lala also enrolled in Sainan High to become closer to Rito. When Haruna toured Lala around the school, another recurring character was introduced: Taizou Motemitsu, the pitcher of the baseball team who's notorious for hitting on girls then suddenly rejected and exclaimed by his posse. By the twentieth chapter, one of Lala's childhood friends, Ren Elsie Jewelria, also enrolled in Sainan High to become closer to Lala. A few chapters later, three more girls were introduced, the arrogant queen bee Saki Tenjouin, and her friends Rin Kujou and Aya Fujisaki, all three being one year higher than Rito and his friends. Later, Ren's female counterpart, Run, also enrolled in the school despite Ren being enrolled there already.
Rito and Lala initially attempted to hide the fact that Lala is an alien. Students would often notice Lala's Devilukean tail swinging behind her, but at first they simply passed it off as some accessory. Eventually, their friends discovered that she is actually an alien, to which they reacted surprisingly unsurprised. And soon enough, much of the entire school knows that she is an alien, also none too surprised.
On Rito and Lala's second year, Yui Kotegawa was introduced, who is a member of the student morals committee, and is often seen reprimanding the students' "shameless" behavior. In the abandoned building next to the school, there lived a ghost named Shizu Murasame, who soon enrolled in the middle of the year after Dr. Mikado gave her an artificial body.
In To LOVE-Ru Darkness, Lala's twin sisters, Momo and Nana, enroll as well as Golden Darkness (Yami). Momo and Nana enrolled in Sainan High because Momo would like to keep an eye on Rito and his girls so that her Harem Plan may succeed (she just convinced Nana to join her in enrolling). Nana soon befriends Mea Kurosaki, who is revealed to be a second generation Transformation Weapon (Yami's little sister), who joined Sainan High a month prior to keep an eye on Golden Darkness.
Right after enrolling, Momo instantly becomes popular, and soon has her own fan club, the Venus Momo Club (VMC). Like her sister, the club knows that Momo is an alien princess from the planet Deviluke. They seem to have nothing better to do than to follow and worship her, which she finds annoying.
The most prominent staff in the series is Ryoko Mikado, the school doctor, who also happens to be an alien. She becomes a close friend and ally to Rito, Lala, and their friends, and they would consult with her whenever they have alien problems. The second most prominent staff is the notoriously perverted unnamed principal of the school. The principal would usually appear only to chase his female students or to offer other cute girls to enroll into his school. The third most prominent staff is Honekawa-Sensei, Rito and Lala's elderly homeroom teacher. He is the only seen teacher who actually discusses something in the classroom. In To LOVE-Ru Darkness, Dr. Tearju Lunatique substitutes Honekawa as Rito and Lala's homeroom teacher before becoming the homeroom teacher of Nana, Momo, Yami and Mea's class. The least prominent staff are Sasuga, the tennis club's handsome advisor, and Naruiwa, a teacher in the disciplinary team.
- There are warning posters of the Principal throughout the school.
- There are also recruiting posters of the VMC throughout the school.
- The school's uniform design is similar to the uniform from the One-Shot Manga Futagami Double, illustrated and authored by Yabuki Kentaro, the illustrator of the To Love-Ru series.
- The name of the high school in To Love-Ru, Sainan (彩南), is a homophone to the Japanese word for catastrophe (災難). This is probably a wordplay related to the theme of the series.