Heh, heres another fact although not related too much to the demon origins.
Dante had "unrequited love" for Beatrice, which is exactly or very simular to Nero and Kyrie along with it being secrective which is what Nero/Kyrie done with Credo, description from Wiki:
"Unrequited love is love that is not openly reciprocated, even though reciprocation is usually deeply desired. The beloved may not even be aware of their admirer's deep affections. This can lead to feelings such as depression, low self-esteem, anxiety, and rapid mood swings between depression and euphoria. Being such a universal feeling, it has naturally been a frequent subject in popular culture. Despite the case of the beloved not knowing their admirer's deep affections, it is common that the subject of unrequited love is a close friend.
That description sums it up.
Anyways back on topic.