Allah is the Protecting Friend of the believers, bringing them from darkness into light. As for those who disbelieve, their friends are the Satans (Shayateen), who bring them out of light and into darkness. These are the people of the Fire, wherein they shall abide forever. (Surah Al Baqarah: 257)
The Arabic word "Wali" refers to an aid, ally and a friend. Allah is the "Wali" of the Believers (Mu'meen), taking them out of the darkness of infidelity (kufr) and polytheism (shirk), and bringing them into the light of Islam. He thus keeps them steadfast in the light and saves them from darkness.
On the contrary, the infidels (kuffar) have the devils as their accomplices who wish that they never emerge from the abysmal darkness of infidelity (kufr) and polytheism (shirk). By continuously preventing them from Islam, the Satans (Shayateen) wish to take these people together with them to Hell where they will all remain till eternity.