"Idol" of Cross Academy, an aristocrat with an icy power, and Kaname's right-hand man.
As a young heir, Aidou was heavily pampered and convinced he was the smartest and cutest little vampire ever; he was nothing short of a spoiled brat. Then, he met a pure-blood - Kaname Kuran. Immediately, Aidou declared hate for the higher class vampire, shocking his father, cousin, and friend. Many reasons as to why are speculated, but it isn't entirely clear why. However, as the two grew older, Aidou regretted his desicion and ended up flipping his entire perspective; he then became Kaname's most loyal and respectful follower.
He has a sister, Tsukiko Aidou.
At the start of the series, Aidou is arguably the most bubbly of all the night class students. Unlike most of the vampires, he enjoys all the attention of the day class students and often banters with the girls, who dub him 'Idol', much to prefects Yuuki and Zero's chagrin. It's very evident that he's not very fond of Yuuki or Zero from the very beginning. Similar to Ruka, he, too, doesn't get why Kaname, his leader, is so intimate with a rather plain human girl. Because of this, he's sometimes a little harsh in his actions towards Yuuki, having once tasted her blood and froze her feet to the floor. Of course, Kaname always finds out and always punishes Aidou with a smack. It isn't exactly necessary because Aidou is very, very loyal (possibly more so than any other) to pure-blood Kaname. His regards for Zero is low due to fact that Zero's regards for Kaname aren't too high and his for Yuuki are much too high.
His aforementioned loyalty to Kaname is first tried when he becomes the sole witness of Kaname's murder of Shizuka Hiou, another pure-blood. He becomes confused and ends up going to Yuuki and Zero for comfort. There, he agrees with Yuuki that he'd follow Kaname even if he were betrayed by him. As for Zero, he finds out his vampiric secret and decides to keep it a secret.
During the Rido Kuran arc, when it's revealed that Yuuki is Kaname's sister, Aidou immediately attempts to treat her with the respect that belongs to all pure-bloods, but seems to have difficulties along the way. During this, Kaname's faith in him is measured as it is Aidou who is asked to watch over Yuuki whilst Kaname takes care of the vampire council.
After the time skip, Aidou has become Yuuki's tutor of sorts and seems to be her main source of companionship, since Kaname is often gone and Yuuki is not yet ready to face the vampire world as a vampire. It is very apparent that their relationship has become much more stable, since now Aidou can very clearly see why Kaname was always so stuck on her. Still, he's a bit informal with her, but also doesn't hesitate to put his life on the line for her when Touma, a pure-blood, threatens her. Even so, they are both injured and it is Zero who ends up patching them up at an HQ of the Hunter's Association. During this, Aidou interestingly and vaguely tells Zero that if he wanted something, he'd take it. This is often interpreted as Aidou's thoughts on Zero's situation with Yuuki.
At said HQ, Aidou becomes a prisoner of sorts and is interrogated in vain and eventually let go. However, he returns shortly after in despair after seeing his father killed by Kaname without reason. Yuuki promises to find out and relieves Aidou of his duties of watching over her. Despite that, Aidou remains adamant in staying and taking care of her. He shows this by 'shockingly' joining the Yuuki-lead reinstatement of the night class, which was dissolved during the Rido Kuran arc.
Though Yuuki has seemingly become Aidou's new favorite Kuran, he states to Zero that he, unlike Zero, is not tormented by hatred for Kaname.