Cheops is the first ESA mission dedicated to the characterization of known transiting exoplanets by means of ultra-high-precision photometry of bright stars. Data from Cheops will allow deriving precise radii for planets all the way down to Earth-size planets. Combined with ESPRESSO, it will lead to the measurement of accurate planetary masses and radii for an unprecedented large sample of exoplanets, thus providing average planet densities, a critical input to the study of their bulk composition and internal structure. These results are, however, strongly dependent on how precise our knowledge is of the host stars. This project addresses, with unprecedented detail, the problem of deriving precise stellar parameters and chemical compositions for the host stars, focusing on the complex case of M dwarfs.