Gandalf, the servant of Secret fire?

[Voralphion]

I always wondered why OLORIN would be the race of FIRE maiar, which is usually associated with demonic and evil nature.

Olorin was not a fire maia, I guess meaning one that is associated with fire such as the balrogs and Arien, but a maia who spent a great deal of time in the gardens of Lorien and in the Halls of Nienna. He was chosen specifically for the quest whereby he became gandalf by Varda and Manwe, so he could have done various tasks for them.

Also, just because fire is usually associated with evil, it doesn't mean it is. Many of the maia of fire were corrupted by melkor but not all. Arien was a fire spirit the Melkor was unable to carrupt, and ended up guiding the sun. She is probably not the only one who remained good, but just that no others were mentioned.