  We present extensive observations and analysis of supernova (SN) 2021dbg,
utilizing optical photometry and spectroscopy. For approximately 385 days
following the explosion, SN 2021dbg exhibited remarkable luminosity, surpassing
most SNe II. This initial high luminosity is potentially attributed to the
interaction between the ejected material and the surrounding circumstellar
material (CSM), as evidenced by the pronounced interaction signatures observed
in its spectra. The subsequent high luminosity is primarily due to the
significant $^{56}$Ni ($0.17 \pm 0.05$ M$_{\odot}$) produced in the explosion.
Based on the flux of flash emission lines detected in the initial spectra, we
estimate that the CSM mass near the progenitor amounted to $\sim$(1.0--2.0)
$\times 10^{-3}$ M$_{\odot}$, likely resulting from intense stellar wind
activity 2--3 yr preceding the explosion. Considering the bolometric light
curve, nebular spectrum modeling, and mass-loss rate, we suggest that the
progenitor of SN 2021dbg was a red supergiant (RSG) with a mass of $\sim 20$
M$_{\odot}$ and a radius of 1200 R$_{\odot}$. This RSG featured a thick
hydrogen shell, which may have contained a region with a sharp decrease in
material density, electron density, and temperature, contributing to its
layered structure. This object demonstrates mixed features of SNe IIP and SNe
IIL, making it as a transitional event linking the above two subclasses of SNe
II.
