Hello I am glad you have read the book of Psalms 91 when you ask who is the secret place I believe it is a Hidden concealment.
Psalm 18:11
Easy-to-Read Version
11 He wrapped himself in darkness that covered him like a tent.
He was hidden by dark clouds heavy with water.
Psalm 27:5
Easy-to-Read Version
He will protect me when I am in danger.
He will hide me in his tent.
He will take me up to his place of safety.
Psalm 31:20
Names of God Bible
20 You hide them in the secret place of your presence
from those who scheme against them.
You keep them in a shelter,
safe from quarrelsome tongues.