To fulfill the desire of a devotee, this un-manifest, all pervasive Brahm (God) by the power of His Yogmaya, manifests Himself as an embodied Being and appears before the devotee. In fact there is no difference between the Manifest and Un-manifest aspects of God. It is the Un-manifest that becomes manifest only.
Brahm (God) manifests Himself in the same form in which a devotee likes to see Him. In Bhagwat Gita, Arjuna wanted to see God's Universal form in the first instance, then His celestial form endowed with four arms, and last of all His human form with two arms only. Divine Lord Krishna, in order to fulfill the desire of Arjuna, appeared before him in all these forms successively within a short time.
So the God appears before a devotee in whatever form he worship Him, whether it is the form of Rama, Krishna, Vishnu, Shiva, Shakti, Ganesha or any other form. A devotee should always remember that the God whom he worships, always pervades through this whole universe & can manifest Himself in any form, anytime He wants to appear before his devotee. God is never invisible to such souls who have understood His true nature, nor does He ever lose sight of them.
Oh God, as you always hear to your devotees, so pl. pl. "Hear Me Lord"