Abstract Kelvin–Helmholtz instability (KHI) at unmagnetized planetary ionopauses plays a key role in driving solar wind–ionosphere coupling and planetary ion escape. We investigate KHI using a two‐dimensional finite Larmor radius (FLR) MHD model. The FLR effect produces hemispheric asymmetry: the sign of Ω⋅B $boldsymbol{Omega }cdot mathbf{B}$, where B $mathbf{B}$ is the magnetic field and Ω=∇×v $boldsymbol{Omega }=nabla times mathbf{v}$ is the vorticity, determines whether KHI is enhanced or suppressed. During nonlinear evolution, the FLR effect suppresses secondary instabilities and turbulent mixing. Long‐wavelength vortices detach as plasma blobs, whereas short‐wavelength vortices remain attached, sharply reducing ion escape. The preferred hemisphere also depends on ionospheric magnetic‐field strength. Under strong fields, KHI develops preferentially for Ω⋅B>0 $boldsymbol{Omega }cdot mathbf{B} > 0$; as the field weakens, the growth rates cross and Ω⋅B<0 $boldsymbol{Omega }cdot mathbf{B}< 0$ becomes dominant. These results highlight ion gyro‐motion as an important control on hemispheric asymmetry and plasma loss at unmagnetized planetary boundaries.