The Belle II experiment investigates the potential interactions between Standard Model (SM) particles and dark sector particles by means of weak couplings mediated by new particles. We present the outcomes of two distinct searches. First, we examine rare $B$ decays in search of a long-lived (pseudo)scalar particle. Then, we provide an update on the ongoing investigation into an invisibly decaying $Z′$ boson. Furthermore, we look into lepton flavor violation by exploring $\tau$ decays into $\ell \alpha$, where $\alpha$ represents a novel, invisible boson. We also report on the first inclusively tagged reconstruction of $\tau$ pair events, focusing on the neutrinoless decays $\tau \to \ell \phi$. Lastly, we present the most precise measurement of the $\tau$ lepton mass worldwide. All of these studies utilize data collected by the Belle II detector during the period of 2019-2021.