Who is Krishna and where did he come from?

Answer 1

Krishna is the Supreme Personality of Godhead, the Supreme Fountainhead of all existence. He has His eternal abode in the spiritual sky beyond the material plane of existence. He descends to re-establish religious principles whenever they become disturbed.

References: Srimad Bhagavatam 1.1.1

Bhagavad-gita 4.7-8

Answer 2

Krishna was not incarneted from any God. He was THE GOD as per Geeta. He has mentioned in Geeta that all the loka ( Places of various gods) till Brahma loka are recurring. Means the person who goes there comes back to earth for rebirth. But the person who reaches to his paramdhama (Lord Krisna's place) never returns back to earth for rebirth. (8/16) & (8/21)

Answer 3

In Gita, it is writ:

" यदा यदा हि धर्मस्य ग्लानिर्भवति भारत |

अभ्युथनमधर्मस्य तदात्मानं सृजाम्यह्म् "

This verse says that, when there is the decline of righteousness on the earth, he comes and uplifts it.