  We present a new model compound with the S = 1/2 frustrated square lattice
composed of the charge-transfer salt (o-MePy-V)PF6. Ab initio calculations
indicate the formation of an S = 1/2 square lattice, in which six types of
nearest-neighbor ferromagnetic- and antiferromagnetic interactions cause
frustration. By applying a magnetic field, we observe an unusually gradual
increase of magnetization and a subsequent 1/2-plateau-like behavior. A
numerical analysis using the tensor network method qualitatively demonstrates
such behaviors and suggests a collinear ordered state and a field-enhanced
quantum fluctuation. Furthermore, the local magnetization and T1^-1 probed by
nuclear magnetic resonance measurements support these findings.
