Many of my friends married recently and all of them were either unhappy or divorced. I was wondering if there is any mantra or verse that ask god to provide very nice wife. Is there any verse to have nice wife?

Both the thousand-armed Hehaya king Kārtavīryārjuna and the ten-headed Rākṣasa Rāvaṇa are said to have conquered the entire universe at some point, subduing even the gods in the heavens themselves; and each one of them was eventually defeated and killed by an avatāra [incarnation] of the god Viṣṇ...

Hinduism acknowledges god as omnipresent - he is present everywhere and in every soul. He is said to have created the cosmos. Then, why did he create demons and does he manifests in demons too?