Yuki & Kaname

Yuki Kuran and Kaname Kuran have a long history and have developed a deep and important, if somewhat at times, mysterious relationship

Background
It is mentioned both in the anime and manga that Kaname and Yuki were engaged to be married since they were childern. This is a common practise among pure bloods to preserve their blood lines and due to the fact that vampires are immune to inbreeding.

History
Kaname once declared that Yuki was born to be his wife. Yuki loved and adored her brother and was very protective of him. Since birth, Yuki was hidden away from the outside, locked in a single room with no windows and Kaname sympathised with her situation. Following the death of their parents and Yuki's new humanity, Kaname was taken into the care of Asato Ichijo, while Yuki was left in the care of Kaien Cross.

On her first day, the Chairman handed her a plate of pudding, but she just sniffed and poked it. Kaname picked up the spoon and tried to feed her, but when he tried to do so, Yuki saw his fangs. She reached up with her fingers and touched them. She became scared as she remembered the Level E that attacked her. Saddened by the fact that Yuki had become frightened of him, he leaves to calm her down. A week later, he receives a message from the Chairman that said Yuki had been attacked by vampires. He rushes over only to realize that he had been tricked. The Chairman then excitedly points out that Yuki had dressed herself, even though the clothes were on wrong. Kaname falls into a fit of laughter as Yuki covers her head, leaving her bottom out. She then looks out at Kaname as he gets a hold of himself. He then apologizes for scaring her, as he is leaving he says "bye bye". But as he is trying to leave, she grabs hold of his coat saying her first word, "Kana…me! Kaname!"

Though Yuki now had no memory of him and even was a little scared of Kaname's vampire side, the ever vigilent Kaname Kuran would ditch his guards and visit whenever prompted as well as on Yuki's anniversary of her being saved by Kaname, which as a child, she referred to as his birthday. The human Yuki grew trusting and fond of the vampire, with a strong belief in his gentleness based upon his gentle caring of her. She soon fell in love with Kaname and was constantly eager for each visit and his attention.

After the creation of the school, Yuki went to visit Kaname. However the vampire was away, as a result she fell asleep on the stairs of the night class's hall, almost bitten by a vampire, Kaname finds the vulnerable Yuki and takes her to his room where unbeknownst to Yuki, almost giving into temptation, he almost bites her. Instead, he ends up biting Ruka in front of her and Yuki runs off in shock. As of that point, some distance is created between them, Kaname hoped this would help her understand the cruel vampire nature.

Summary
Later in the series, Kaname is forced to bite Yuki to save her from the danger of Rido Kuran. Kaname "awakens" her vampire side and listens as she remembers he is her brother.

At first, the thought of being in love with her brother repulses her, but then Kaname reveals that their mother and father were brother and sister and that someday they will be just like them.

Their Relationship Before Yuuki's Awakening
She does not seem shocked when she learns Kaname is a Pureblood, she thinks it will not change her opinion of him. After all, she admires and idolizes him. Kaname loves Yuki more than anything in the world. He is always protecting Yuki and deems her as the most important person in the world to him. He has said many times that Yuki is "his dear girl".

Since he is feared by the other vampires because he is a Pureblood and because Yuki had all of her memories when she was five years old erased, he is often very lonely. Few people are close to him. Yuki cares about Kaname so much, her heart beats like crazy when he’s around her. She loves Kaname while he himself adores and cherishes her.

However once more, Kaname is a vampire and what is more important, a pureblood. He cannot allow himself to step closer to Yuki and risk to hurt her...or even curse her forever by drinking her blood. This one fact demonstrates just how important Yuki is to Kaname, but it does not hold. As danger settles in on the academy, Kaname feels that the way to keep the always-in-danger Yuki from being more of a vulnerability than needed is to reveal her true nature as a pureblood.

As a child, Yuki always referred to Kaname as her "Onii-sama" (Respectful and formal Japanese for elder brother), following her turning human, she at first called him "Kaname" (informal) and later "Kaname-sama" (respectful and formal) as taught by Kaien. Later at school, she referred to him as "Kaname-senpai" (Respectful Japanese to refer to older students). Yuki revealed in 25th Night that she had been determined not to refer to Kaname as "Kaname-sama" in a deliberately attempt to distance herself from Kaname, but accidentally finds herself using it. Following her re-awakening, Yuki returns to calling Kaname "Onii-sama" and occasionally "Kaname-senpai" despite his repeat requests to simply call him "Kaname".

Their Relationship After Yuuki's Awakening
After finally telling Yuuki that he is not her real brother, Yuuki drinks his blood to see his memories. After viewing the horrifying death of the real brother and how the ancestor reverted himself into a baby, Yuuki gently reassure Kaname that the only Kaname she knows is him anyway and that he can tell her anything without holding back. She even refers to Kaname without any honorifics like he always wished.

Yuuki tells Kaname she wants to know about his past, not just about the awakening but his past as the ancestor. As she goes deeper into his memories she sees them as children again in the snow after the death of their parents. They were being pursued as they escaped and Kaname who was carrying an unconscious Yuuki, set her down so that he could fight the enemies so he could take her to Cross' place for Yuuki's safely. Kaname looks so sad as he says that the next time she wakes up, she'll look at him with the eyes of someone that doesn't remember him.