Kyouya Takamachi

Triangle Heart: Kyouya is the son of Takamachi Shirou, and is the heir to his father's Mikami twin kodachi style. He lives He has vowed to avenge his father who was killed when he was young. He is extremely close with Miyuki, who grew up with him, and he considers her his blood sister even though they're cousins. Although Kyouya is the better swordsman, the team make an excellent team in a fight. He is hired with Miyuki to defend his childhood friend, Fiasse, from a threat to kidnap her. Ellis McGaren, Fiasse's official hired bodyguard and also Kyouya's childhood friend, is initially unhappy with both of them working together to protect Fiasse, and thinks Kyouya will just get in her way.

Eventually the two learn to get along and work together. Mahou Shoujo Lyrical Nanoha: Kyouya is the real brother of Miyuki and Nanoha here, and lives with his mother, Momoko, and his father, Shirou. Him and Miyuki both play a very minor role in the Nanoha series, but they are both still excellent swordsmen. Kyouya is Miyuki's teacher in the series. In one episode, Kyōya takes Nanoha to the Tsukimura mansion and spends the day with Shinobu, the two a very discreet couple. He remains unaware of Nanoha's magic powers throughout the series, until the end of A's when Nanoha reveals it to him and the rest of her family. (source: Wikipedia)