This is Chapter 21 from the Ouran High School Host Club manga series.
After a summer vacation riddled with various plot twists, the Host Club is back at school! Haruhi and the guys are shocked to see an unusual guest; that is, a little girl who immediately tags them as a male harem. At first, Tamaki and the twins believe they've misheard her, until she cries, "Debauchery!" and then proceeds to name each Host Club member: Kyouya the megane, Mitsukuni the lolita, Takashi the stoic, Haruhi the nerd, and the twins are called incest.As she stares at a bewildered Tamaki, she suddenly calls out, "Brother," and leaps into his arms. The Host Club look on in wonder, asking Tamaki why he never told them he had a little sister, with Tamaki himself protesting that he thought he was an only child, too. When they ask the little girl what her name is, she responds with "Kirimi," which Haruhi mistakes for "creamy," while the twins (in typical devil's advocate fashion) ask if she said, "Kill me." Tamaki denies being her brother, but upon seeing her tear up, his pushover self quickly tells her that he will be her brother as of today. The Host Club immediately discusses what to do, offending Tamaki by calling him rash.
Suddenly, Umehito Nekozawa (President of the Black Magic Club) appears, shocking everyone and after hearing Kirimi's surname, the Host Club realizes that she is his little sister. With a shock of realization, Haruhi states that Umehito is actually blond under his wig, but Kirimi cowers in fear when facing Umehito. The twins try taking Umehito's dark costume off of him, but he screams, since he hates the light. Miraculously, two family servants of the Nekozawa family appear and state that while Umehito hates the light, Kirimi hates the dark.
The servants go on to explain that the Nekozawa family members are descendants of the Russian Tokarev (corrected by Tamaki as Romanov) Dynasty. Legend has it that every few hundred years, a member is born that is drawn to darkness, with this being Umehito. Kirimi is naturally scared of her darkly dressed brother, and has dreamed of meeting her blond haired, princely brother, which she has only seen in a portrait. The maid continues to explain that she tries to help her feel safe by reading Kirimi tales of princes, but when she ran out, the maid read her shojo mangas thus revealing the cause of Kirimi's labeling vocabulary towards the Host Club.Haruhi has trouble with Kirimi's name and mistakes it for "Sashimi" when asking Umehito if Kirimi knows that he's her brother, whereupon the twins hasten to correct her by stating that the girl's name is "Creamy," which is also incorrect. Umehito simply responds by saying that he prays for Kirimi to love the darkness, and the Host Club members chastise him for trying to force that on his sister, saying that it should be the other way around. As the servants try to get Kirimi to go home, she clings to Tamaki, saying that she wants to stay with her "brother." Umehito grows saddened and after asking Tamaki to dote on Kirimi for him, runs off, black cape and all.
After hearing Kyouya state that Umehito looks up to him for being Kirimi's ideal brother, Tamaki grows determined to patch up the siblings' relationship and orders the Host Club to make such a thing possible, with demands like darkening the room and such. Tamaki apologizes to Kirimi, saying that he isn't her brother, after all and strikes a pose with the the Host Club, yelling that they're the Ouran Host Club Police Department. After dragging Umehito back, Tamaki announces that he will make Umehito love the light little by little and that he will shape him into Kirimi's ideal prince character of a brother.And so, Umehito's hardcore training camp begins. Umehito is seen reciting beautiful sounding phrases and compliments, until he messes up at the end and says "as evil as a cursed wax doll," causing Tamaki to rage and demand stricter instruction. The twins look on in wonder and note how spartan-like Tamaki has become as he drills Umehito in the gracious ways of being a proper brother and gentleman. Tamaki procures a practice partner (a teddy bear, a Haruhi doll, etc.) and demands that Umehito sweet-talk it. As time passes, Tamaki works on Umehito, while juggling pleasing Kirimi with sweets and such, and Haruhi is forced to read shojo mangas to Kirimi. Finally, it is shown that Umehito can stand a flashlight's beam. However, Kirimi enters at an unlucky time and when Umehito turns to look at her, he has the flashlight beam shining up into his face, causing Kirmi to panic at the creepy looking visage and to flee.
The twins note that she's in the courtyard below, looking at a cat. Umehito dithers and runs to the window, claiming that Kirimi is terrified of cats. Without his robe on, Umehito bursts out to save Kirimi in broad daylight, comforting her before falling because of too much sunlight exposure in one short moment. It is later shown that while Umehito reverted to his old state, Kirimi is able to accept her brother and the two siblings connect. The chapter ends with Kirimi calling Tamaki a liar for being a horrible brother impersonator, leaving Tamaki flabbergasted.