We present two results of a search for MeV-scale neutrino and anti-neutrino events correlated with gravitational wave events/candidates and large solar flares with KamLAND.

The KamLAND detector is a large-volume neutrino detector using liquid scintillator, which is located at 1 km underground under the top of Mt. Ikenoyama in Kamioka, Japan.
KamLAND has multiple reaction channels to detect neutrinos. Electron antineutrino can be detected via inverse-beta decay with 1.8\,MeV neutrino energy threshold. All flavors of neutrinos can be detected via neutrino-electron scattering without neutrino energy threshold.
KamLAND has continued the neutrino observation since 2002 March.

We use the data set of 60 gravitational waves provided by the LIGO/Virgo collaboration during their second and third observing runs and search for coincident electron antineutrino events in KamLAND.
We find no significant coincident signals within a $\pm$500\,s timing window from each gravitational wave and present 90\% C.L. upper limits on the electron antineutrino fluence between $10^{8}$--$10^{13}$\,cm$^{-2}$ for neutrino energies of 1.8--111\,MeV.

For a solar-flare neutrino search at KamLAND, we determine the timing window using the solar X-ray data set provided by the \textit{GOES} satellite series from 2002 to 2019 and search for the excess of coincident event rate on the all-flavor neutrinos.
We find no significant event rate excess in the flare time windows and get 90\% C.L. upper limits on the fluence of neutrinos of all flavors (electron anti-neutrinos) between $10^{10}$--$10^{13}$ cm$^{-2}$ ($10^{8}$--$10^{13}$\,cm$^{-2}$) for neutrino energies in the energy range of 0.4--35\,MeV.