In stellar flares, where high-temperature plasmas exceeding several million Kelvin can be generated, X-ray spectroscopic diagnostics have provided crucial observational insights. 
In particular, X-ray observations covering a broad energy range of 1.7-12 keV, which includes K-shell transitions of key elements from Si to Ni, along with an energy resolution of R = E/dE ~ 10^3 had never been achieved until recently, even for solar flares. Such observations have now become possible for the first time with XRISM, enabling us to better resolve this eruptive phenomenon, for example in terms of plasma kinematics, non-thermal properties, multi-thermal structures, and chemical compositions.

There have been two observations of RS CVn-type binary stars during the performance verification phase, serving as excellent showcases to demonstrate XRISM's capabilities and to anticipate future large flare observations. Fe XXIV-XXVI K-shell lines from stellar coronal plasmas were successfully resolved for the first time. One target is GT Mus. Two thermal components with temperatures of kT~1.7 keV and 4.3 keV were derived from the analysis using five combinations of Fe XXIV-XXVI K-shell main and satellite line ratios, as well as from the thermal broadening of the Fe XXV K and broadband fit over the energy range of 1.7-10 keV. Additionally, scenarios involving non-equilibrium ionization and non-Maxwellian electron energy distributions were ruled out. The other target is HR 1099, which captured a complete flare event. Using Fe XIX-XXVI K-shell lines, differential emission measure (DEM) distributions over a wide temperature range, approximately 1-10 keV, were reconstructed, revealing a significant enhancement in the kT~5 keV components during the flare. Furthermore, elemental abundances of Si, S, Ar, Ca, Fe, and Ni were derived, with notable increases in Ca and Fe abundances during the flare. These enhancements were found to be consistent with the standard chromospheric evaporation scenario in the context of the first ionization potential (FIP) effect.