Triangle Heart:
Miyuki is actually the cousin of Kyouya and Nanoha, but is raised by their family and she considers them siblings. She is very close to Kyouya, and an accomplished Mikami twin kodachi style swordsman like her brother. She and Kyouya are hired to guard their childhood friend Fiasse, who is now a legendary singer and heiress. Fiasse's real bodyguard, Ellis, is particularly set off by this, and the three guards must learn to deal with and respect each other as they protect Fiasse on her concert tour around the world from someone who seeks to kidnap her. Miyuki and Kyouya work particularly well together, and she is shown as very competent on her own, defeating an entire Special Duties Unit team single-handedly in the first OVA. However, Miyuki does not like to fight, seeing it as simply a duty to protect those one cares about.

Mahou Shoujo Lyrical Nanoha:
Miyuki is the real sister of Nanoha and Kyouya here, and lives with her two siblings, her mother Momoko, and her father Shirō. She is a very minor character, but her and Kyouya are still extremely close, and are still excellent swordsmen. In one episode, Nanoha, wishing to improve her technique with her magical staff Raising Heart, observes Miyuki's solo training. Throughout the first season, Miyuki remains unaware of magic, although she founds out about Nanoha's powers at the end of A's.

(Source: Wikipedia)