2025ApJ...989...53J

We present BVi multiband high-cadence observations of a Type II supernova (SN) KSP-SN-2022c from a star-forming galaxy at z ≃ 0.041 from its infant to nebular phase. Early light-curve fitting with a single power law is consistent with the first detection roughly 15 minutes after shock breakout (SBO). The SN light curves feature a rapid rise and decline across its luminous (V ≃ -18.41 mag) peak together with a short plateau. The presence of the short plateau and rapid postpeak decline place the SN within a small group of transitional type between Type II-P and II-L subtypes. Its broad and asymmetric H profiles with large emission-to-absorption ratios and its near-peak luminosity in excess of predictions from models of SN shock cooling both point to circumstellar interactions in this SN. Early colour evolution exhibits a short-lived blueward motion in B - V within the first few days and continuous reddening in V - i, inconsistent with simple blackbody heating. Our simulations of SN light curves estimate 13 M_⊙ and 680 R_⊙ for the mass and radius of the progenitor, respectively, together with circumstellar material (CSM) of 0.73 M_⊙ to account for the excess luminosity and rapid postpeak declines. We discuss the origin of its short plateau and early colour evolution in the context of partial envelope stripping of the progenitor star and a delayed SN SBO near the edge of the CSM, respectively, as indicated by our simulations. We establish a correlation between postpeak decline rates and CSM mass in Type II SNe, highlighting that CSM interactions play a major role in shaping the postpeak evolution of transitional types.