  We study the hybrid formulation of Green-Schwarz superstrings on AdS_3 with
NS flux and the boundary N=4 superconformal algebra. We show the equivalence
between the NSR and GS superstrings by a field redefinition. The boundary N=4
superconformal algebra is realized by the free fields of the affine Lie
superalgebra A(1|1)^{(1)}. We also consider the light-cone gauge and obtain the
N=4 super-Liouville theory which describes the effective theory of the single
long string near the singularities of the D1-D5 system.
