Mikado Ryōko

Ryoko Mikado is the school doctor at Sainan High. She also helps out other extraterrestrials on Earth in various manners, usually those dealing with medical care, though she does offer students a variety of her own concoctions.

Personality
Formerly an operative for a shadowy, black market extraterrestrial organization bent on overthrowing with Emperor Gid, she fled from her superiors to Earth in hopes of hiding her identity. Proof of her non-human origins are the elf-like ears hidden by her short hair, and her extreme weight, though it has not yet been specified what her species is. Furthermore, she seems to be smitten with Rito, shown when she once asked him to rub oil on her at the beach, and didn't look surprised when Rito accidentally says love you to her, Nana, Yui, and Run.

She also shows that she is comfortable enough to converse with Rito in a serious and mature manner (yet still obviously caring since she was thinking of giving him advice even when nobody asked her to do so, albeit she never came up with one in the end), as seen when she approaches him one on one inside his room to say that she likes the planet Earth and that Rito must do what he can to save it while enclosing his hand on both of hers seemingly comfortable in telling him her serious feelings and thoughts about the matter. It is also notable that she doesn't react shy nor negatively affected by the fact that he was obviously having a perverted imagination during the middle of the conversation (a trait of Rito that characters of the series (one of which is Saruyama) notice whenever he's embarrassed or thinking of something ecchi), merely ignoring it even with his face visibly steaming red. Ryoko also has no problem with Rito seeing her almost/naked with her clothes being melted by the king of Deviluke's plant traps or during their trip to the hot springs (where they shared a red hot spring bath face to face with the inclusion of Saruyama).

Rito Yuuki
Mikado-sensei works as the doctor at Rito's school. Rito would occasionally come by her clinic and observe her working with other aliens living on Earth. Rito and the others have a good and trusting relationship with Mikado, and would occasionally come by her nurse's office at school to socialize or talk about important matters, such as Golden Darkness, Tearju, and the organization they were associated with, which Mikado is able to investigate through her former black market connections.

Mikado is quite fond of Rito and is amused by his innocence. She can be quite mischievous with him, and likes to have fun with his shyness around women, such as asking him to take off Yami's clothes so Mikado can treat her sickness, making him rub oil on her at the beach, and instructing Lala to add aphrodisiac to her Valentine chocolate for Rito.

Oshizu Murasame
Mikado built the artificial body that Oshizu inhabits so that she may live like a normal physical girl. Oshizu now lives with Mikado and works at her clinic as her nurse, though she can be quite clumsy sometimes.

Tearju Lunatique
Tearju and Mikado have been friends since they were in school together. They went their seperate ways some time before Tearju was recruited into the suspicious organization where she would eventually create Eve (Golden Darkness).

In To Love-Ru Darkness, Mikado eventually reunited with Tearju to tell her how Eve is doing on Earth. It was because of Mikado that Tearju came to Earth and became the homeroom teacher for Rito's class.