Onii-Chan's means of control

Kagura Marie is the younger sister of Kagura Yuuji  in the ani-manga series Tona-Gura!. She and her brother once lived next door to the Arisaka sisters, Kazuki and Hatsune, and the two sets of siblings were inseparable till the Kaguras moved away. After ten years of frequent moves that often left the Kaguras without friends outside themselves, they moved back next door to the Arisakas. To the surprise of Hatsune and the great shock of Kazuki, Yuuji was no longer the young gentleman she recalled and crushed on, but a grabby pervert, with former crybaby Marie seemingly an emotionless, heavily armed regulator of her older brother's behavior.

As obnoxious as she found Yuuji's behavior, Kazuki was nearly terrified by Marie, who not only 'punishes' her brother for his obvious transgressions, but tends to read almost any attention Yuuji gives other girls in the same way. Her assault machine gun fires paint balls, but the pain Yuuji gets from these lessons is quite real. This regimen is said to be on the orders of their absentee father, so one can look at it two ways : Either he is genuinely concerned about his son's often-obnoxious behavior, and has entrusted Marie to keep him in line, or their father has abandoned them entirely, and lazily wants his pre-teen daughter to control the son he can't be bothered to instruct on how to behave. Either way, both siblings are united in their anger and sometimes disgust over their father's frequent relocations and absence from their lives. On this front, Yuuji is even protective of Marie. At times, it seems that Yuuji simply accepts his punishment as part of life. He has been shown as being able to easily disarm Marie when she chooses a target other than himself, including the overly affectionate Isokawa Nina and Kazuki herself, once she and Yuuji started a relationship.

Hatsune eventually learns that Marie's behavior is based less in her concern for his 'perverted' nature, and more out of jealousy. Marie misses the brother she had before he hit puberty, when she was the only girl in his life. In both the anime and the manga, Marie undergoes a deep depression when Yuuji backs off from his grabby behavior in order to try and win Kazuki. The manga sees a bit of a rebound when she finally accepts the friendship of the openly affectionate Nina, plus Marie still gets her licks in. Once, when waking her brother up, his hand accidentally touched her chest, which she used as an excuse to open fire. She joins the other ladies in not caring for Yuuji's photography boss, Tojo Haya, and has threatened her as well.

Even with Kazuki and Yuuji starting a relationship in earnest, Marie is not likely to relinquish her place in her brother's life, and for Yuuji, that likely means more punishment for some reason or other.