Yuki is an energic girl full of will to live, always ready to help the others and to fight to protect the important things.
At the begin of the story she's a member of the Day Class and she doesn't remember her past, but even if she has no memories, she lives her life looking at the future, doing her best in the present.
Yuki functions as guardian of the Cross Academy, and together with Zero Kiryu, she does her best to protect the peace of the cohabitation of humans and vampires, that live and study there as Day Class and Night Class.
She has a crush on Kaname Clan, the major representative of the Night Class, that is also her saviour, because he saved her from a bloody vampire when they were children. So he represents her first memory since she can remember.
Zero is also important for her, because they grew up together, and he's the one that more than any other knows Yuki's heart. Usually they spend the major part of the day together, but because of Zero's strong, tsundere personality, they always finish to have a discussion!
Anyway, they're linked by the common thread of destiny~
Kaname and Zero are the two most important persons for Yuki and we can say that her life and her choises are strongly affected by their existences, but it's not just this. She has a lot of friends that she loves and that she protects with all herself. Her heart is pure and her will is absolute, with or without Kamane and Zero.
Yuki will never be changed by someone, because she'll never accept a destiny that other people would decide for her! That's why even if she doesn't know who really she is or what is right or wrong to do, she'll always choose by herself who she wants to be, now and in future.
In fact, Yuki always try to believe in herself, because since she's surrounded by vampires she never know if what she finds is "the truth" or "the truth that the others want she knows".
This is the reason why, when she remembers everything thanks to Kaname, she doesn't know what and who she can trust.
She's Yuki Clan, Kaname's little sister, daughter of Juri and Haruka Clan, pure vampires that years ago were the most important people in the Realm of Vampires.
The reason why her memories were soothed are about Reed Clan, her uncle and the vampire convicted of the death of Yuki and Kaname's parents.
He didn't agreee about the cohabitation of humans and vampires, and Juri and Haruka tried to face him sacrificing themselves, but Reed didn't die, he continued to live in Kaname.
Yuki finds a truth that wil, change her life forever, and she'll force herself to live together with vampires in their world, leaving everything in the Earth, even her best friend, Yori-chan.
Her major sacrifice was renuncing to Zero, the person that she understands to love only when they face again each other as enemies. But when Yuki understands that Kaname wanted to make her as a total, different person than she was, she turns against him.
Zero and her understand their feelings of love, but Yuki won't let Kaname die, or to be alone. Simply, she'll choose to fight in her own way, like the Yuki that she's always been. Her pure heart and her strong wish of happiness for everyone will always make her able to fight with a strengthen that only who believes in tomorrow can obtain~
Zero Kiryu is unique being both a vampire hunter and a vampire. Yuki's close friend and a Cross Academy Guardian, he was trained to be a vampire hunter by Toga Yagari. Zero lives with Kaien Cross and Yuki, after being bitten by a Pureblood vampire, Shizuka Hio, who also murdered his family. Due to the attack, Zero developed a deep hatred and distrust of vampires, despite originally being a gentle character by nature; however, he retains his kindness beneath his scowls, always looking out for the Day Class students in case the Night Class "misbehaves". He has a tattoo on his neck that allows him to be restrained by a charm that temporarily immobilizes him and allows him to enter the Hunters' Association's headquarters. Zero also holds a deep hatred for Kaname Kuran, who tells him that he is simply a shield for Yuki. Zero is deeply in love with Yuki, but chooses to hide it; his feelings with left temporarily confused after Yuki was revealed to be a Pureblood vampire, but Zero continued to love her. In the anime's final episode, Zero lets her drink his blood, allowing Yuki to see how he feels about her. He is one of the few vampires whose body refuses blood tablets, leaving him constantly thirsty for blood; Yuki often gives him her blood to satisfy his thirst. After the time skip, Zero is being considered for the position of president in the future for the Association, and he also drinks the blood of vampires he hunts to satisfy his thirst. Matsuri Hino described Zero as exemplifying her hesitant nature.
Zero's Information Source: Wikipedia
Yuki and Zero are my favorite characters from Vamire Knight and I always supported their love. To be honest, I'm pretty disappointed about the end of the series, because it's like Yuki's love for Zero was putting as the same as her one for Kaname, that is her brother. Yuki said she loves Zero, and Zero loves her, but Dear God... a happy ending was everything but what happened about them. Yuki and Zero spent almost whole their lives together, since their childhood. They became prefectors of the Cross Academy and its protectors, the ones who took care of students of Day and Night Classes.
I loves the moment they spent together like this, because it was there that he bit her for the first time. It was so... exciting. And since Zero is a big tsundere, from there a lot of problems with her. Because he never wanted to admit his feelings, but still... his actions spoke for him. And Yuki... she always was ambiguous abut her feelings, but at the end she said she loves Zero. I think she always looked at Kaname with admiration and as an adolescent does with a boy "prince-like". Anyway, her bond with Zero is what always made me love this series, and I think that they match each other better than anyone else. Because Zero was there when Yuki needed him, and that's the same for her. His fear to becaome a Level E vampire and her help, her support... her love, were the reasons because Zero was saved by her will and her prayers. Also he always protected her, and even if their relation started like one between a half-vampire and a human, at the end the truth is that it was between a girl and a boy, and the rest doesn't really matter. Both of them are vampires and both of them lost their lives at the end, but... their bond is something that will survive forever. I like to think that now it's called eternity.