This study presents the design of a flexible compact rat-race coupler operating at 0.72 GHz on a 1 mmthick paper substrate. The ultra-wide stop-band effect caused due to the combination of the modified stepped impedance stub and the interdigitated resonator slot, offers suppression up to the 14th harmonic. The slow-wave effect caused due to the resonator slot leads to 59.6% size reduction compared with the conventional coupler at the design frequency. The signal transmission through the coupler for different substrate deformation steps has been investigated. Stable responses are reported for the same, making the prototype suitable for flexible electronics applications.