In the wave one Monster High series, Kieran is only referred to as an "emotional vampire". Presumably, this is a vampire that feeds on emotions as well as blood. His preferred emotion is love. But could there be more to the story?
My theory is that Kieran Valentine is actually a Geancannach, a creature associated with the Sidhe of Ireland, such as the Banshee.
Unlike some Sidhe, the Geancannach is not usually associated with the undead, and traditionally, it shows little vampire-like qualities. However, they definitely feed on love.
In Ireland, the Geancannach appears as a charming gentleman. He is very flirtatious, as Geancannach is Gaelic for Love-Talker, or Flirt. However, he is dangerous to young women, as his skin is toxic, making the woman fall madly in love with him. The Geancannach drains her of her love. He disappears once he has his fill of affection, and the woman usually dies from a broken heart.
Ok, so Kieran's methods may be a little different than the traditional Geancannach, but Monster High's version is not the most unusual or off-base take I've seen ( Such as in the TV show Merlin, where it's shown as a magic-eating slug.)
The Geancannach's female counterpart, the Leanansidhe, is much more likely to be described as vampiric. In exchange for love, and sometimes blood, she gives inspiration to her boyfriends, making them poets, musicians, or artists.
In Monster High, Kieran may have been described as being from Transylvania, but his name is definitely Irish Gaelic, meaning "dark one" or "dark-haired".
Another interesting thing is that Kieran and Whisp were sold together at Comic Con, and both the Geancannach and Djinn are associated with "Smokeless Fire". Geancannach are said to be able to generate this smokeless fire.
Personally, I would love this take on this folkloric character.