  An explicit $\textit{spatial}$ localization of a hole is shown in a two-leg
$t$-$J$ ladder in the presence of a staggered chemical potential, which still
retains a translational symmetry, by density matrix renormalization group
method. Delocalization can be recovered in the following cases, where either
the hidden phase string effect is turned off or a finite next-nearest-neighbor
hopping $t'$ is added to sufficiently weaken the phase string effect. In
addition, two holes are always delocalized by forming a mobile bound pair, in
contrast to the localized single holes, which points to a novel pairing
mechanism as one of the essential properties of a doped Mott insulator.
