blue exorcist saison 2 suite saison 1 rent a girlfriend season 3 episode 8
One of the most overlooked aspects of the original series was the nomenclature given to the homunculi, and although their names and purpose have been "clarified" by the manga and Brotherhood, the writers for the original adaptation didn't have this knowledge, so they actually made them work in a completely different way. The whole deal with the Seven Sins is very different in the first anime, as the writers used the homunculus to highlight the aspect of obsession throughout the series. This is why the first anime adaptation had them being "born" in a particular manner, rather than the more trite reasoning given in the manga and Brotherhood much later.
The homunculi are effectively born from the obsession of humans, a theme which is also present in Arakawa's version of the story, even though it has been downplayed a lot.
So what does this have to do with the characters? Well, rather a lot actually. Throughout the whole series, there are very few characters who don't show any of the visible signs that one would normally associate with obsessive behaviour, and this is because they're cleverly hidden for the most part.