Arien

We talked about this in our episode on the Two Trees and the making of the Sun and Moon.

But the Maia that was selected to steer the Sun after the Two Trees were destroyed was named Arien.

And we learn this about her in The Silmarillion: "Arien was chosen because she had not feared the heats of Laurelin," that was one of the trees, "and was unhurt by them, being from the beginning a spirit of fire, whom Melkor had not deceived nor drawn to his service.

Too bright were the eyes of Arien for even the Eldar to look upon; and leaving Valinor she forsook the form and raiment which like the Valar she had worn there, and she was as a naked flame, terrible in the fullness of her splendor." And if you ever needed an example of why good is more powerful than evil in Arda, Arien is a great example.

While the demons of flame and shadow were definitely intimidating, listen to what The Silmarillion says about a Maia of fire that was not corrupted by Melkor.

"And Arien, Morgoth feared with a great fear, but dared not come nigh her, having indeed no longer the power.

For as he grew in malice and sent forth from himself the evil that he conceived in lies and creatures of wickedness, his might passed into them and was dispersed, and he himself became ever more bound to the earth, unwilling to issue from his dark strongholds.

With shadows he hid himself and his servants from Arien, the glance of whose eyes they could not long endure, and the lands near his dwelling were shrouded in fumes and great clouds." I mean, she's so powerful that his servants literally had to hide from the light of her, and Morgoth feared her.