Flexible supramolecular cation structures, [(XyDA)(0.5)(crown ethers)](+) complexes, were introduced as countercations to [Ni(dmit)(2)](-) anions, which bear an S = 1/2 spin, to form novel magnetic crystals (XyDA = p-xylylenediammonium, dmit(2-) = 2-thioxo-1,3-dithiole-4,5-dithiolate, and crown ethers = [12]crown-4, [15]crown-5, or [18]crown-6). Sandwich-type [(XyDA)(0.5)(crown ethers)](+) structures were formed through hydrogen-bonding interactions in the crystals between the ammonium moieties of XyDA and the oxygen atoms of the crown ethers. [12]Crown-4 yielded three types of [(XyDA)(0.5)([12]crown-4)](+)[Ni(dmit)(2)] polymorphs, whereas two types of [(XyDA)(0.5)([15]crown-5)](+)[Ni(dmit)(2)] polymorphs were obtained using [15]crown-5. Three types of [(XyDA)(0.5)(crownethers)](+) structures were identified for these polymorphs. Since the flexible -CH-)-NH3+ moieties connected to benzene plane have rotational freedom, the [(XyDA)(0.5)(crown ethers)](+) structures varied according to the conformations of the XyDA cation in the crystals. The larger ring size of [18]crown-6 than that of [12]crown-4 and [15]crown-5 reduced the flexibility of the XyDA cation, and provided only one type of crystal. The [Ni(dMit)(2)](-) anion existed as dimer structures, which are in the singlet ground state. The arrangement of the [Ni(dmit)(2)](-) anion in the crystals were depended on the structure of [(XyDA)(0.5)(crown ethers)](+). Correlation of the absolute values of magnetic exchange energy J to the square of the intradimer transfer integral t(2) were confirmed.