Comparisons of the peak timescale and peak optical luminosity for various optical transients with t ≳ 1 d. The three blue-shaded regions, from dark to light, correspond to kilonova models associated with SMNS, HMNS or NS–BH, and prompt-collapse scenarios, respectively (Kawaguchi et al. 2020b). The yellow-shaded region indicates the parameter space for central-engine-powered kilonovae (i.e., the so-called “mergernova” model; Yu et al. 2013; Ai et al. 2025). The brown and green shaded regions denote core-collapse and type Ia SNe, respectively (Kasliwal 2012; Cenko 2017; Inserra 2019). The red and pink shaded regions represent the WD–NS mSNe explored in this work, corresponding to models with fiducial and reduced values of f_ej and f_Ni, respectively. For reference, we also indicate three well-known kilonova events associated with SGRB 170817A (red star; Villar et al. 2017), LGRB 211211A (orange diamond; Yang et al. 2022), and LGRB 230307A (green circle Levan et al. 2024), as well as the candidate WD–NS merger event AT2018kzr (purple cross; McBrien et al. 2019).