To understand who he is, what his nature is, what his function is, and so on, we first need to understand what an enlightened being is. Generally, 'being' means any being who experiences feelings pleasant, unpleasant, or neutral. A sentient being, or living being, is a being whose mind is afflicted by the darkness of ignorance. An enlightened being is a being who is completely free from the darkness of ignorance. Just as sentient beings have many different aspects, so do enlightened beings.
Enlightened beings emanate countless different forms for the benefit of living beings.
Sometimes they appear as Deities, sometimes as humans, sometimes as non-humans. Sometimes they appear as Buddhist Teachers, sometimes non-Buddhist Teachers, sometimes as crazy people or evil people, and sometimes even as inanimate objects. Emanations of enlightened beings pervade the whole world, but because our mind is covered by ignorance we do not recognize them. We cannot say who or what is an emanation of a Buddha. Medicine Buddha is an enlightened being who has unbiased compassion for all living beings.