Nature
"In different parts of the universe there are demigods and demons, and in human society also some people resemble demigods whereas others resemble demons. If according to astrological calculations there was conflict between a godly and a demoniac nature, the marriage would not take place." (Srimad Bhagavatam 9.18.23 Bhaktivedanta purport.)
The divine and demoniac natures are described by Lord Krishna in the Bhagavad-gita. "Those who are born with divine qualities follow a regulated life; that is to say they abide by the injunctions in scriptures and by the authorities. One who does not follow the regulative principles as they are laid down in the scriptures and who acts according to his whims is called demoniac or asuric." (Bhagavad-gita As It Is 16.6 Bhaktivedanta purport).
A person who is interested in God consciousness and spiritual advancement is divine by nature, while one who is inimical to God is understood to possess a demoniac nature. Divine and demoniac natures do not mix together well in marriage.