  We demonstrate that an extremely small but positive quantum correction, or
the Casimir energy, to the cosmological constant can arise from a massive bulk
fermion field in the Randall-Sundrum model. Specifically, a cosmological
constant doubly descended from the Planck-electroweak hierarchy and as minute
as the observed dark energy scale can be naturally achieved without fine-tuning
of the bulk fermion mass. To ensure the stabilization of the system, we discuss
two stabilization mechanisms under this setup. It is found that the
Goldberger-Wise mechanism can be successfully introduced in the presence of a
massive bulk fermion, without spoiling the smallness of the quantum correction.
