  We present a spectroscopic/photometric analysis of the rapid rotator
KIC8429280, discovered by ourselves as a very young star and observed by the
Kepler mission. We use spectroscopic/photometric ground-based data to derive
stellar parameters, and we adopt a spectral subtraction technique to highlight
the chromospheric emission in the cores of Halpha, CaII H&K and IRT lines. We
fit a robust spot model to the high-precision Kepler photometry spanning 138
days. Model selection and parameter estimation is performed in a Bayesian
manner using a Markov chain Monte Carlo method. We find that KIC8429280 is a
cool (K2V) star with an age of ~50 Myr, based on its Li content, that has
passed its T Tau phase and is spinning up approaching the ZAMS. Its high level
of chromospheric activity is indicated by the radiative losses in CaII H&K and
IRT, Halpha, and Hbeta lines. Furthermore, its Balmer decrement and the flux
ratio of CaII IRT lines imply that these lines are mainly formed in
optically-thick sources analogue to solar plages. The analysis of the Kepler
data uncovers evidence of at least 7 enduring spots. Since the star's
inclination is rather high, ~70{\deg}, the assignment of the spots to the
northern/southern hemisphere is not unambiguous. We find at least 3 solutions
with nearly the same level of residuals. The distribution of the active regions
is such that the spots are located around 3 latitude belts, i.e. the equator
and +-(50{\deg}-60{\deg}), with the high-latitude spots rotating slower than
the low-latitude ones. The equator-to-pole differential rotation ~0.27 rad/d is
at variance with some recent mean-field models of differential rotation in
rapidly rotating MS stars, which predict a much smaller latitudinal shear. Our
results are consistent with the scenario of a higher differential rotation,
which changes along the magnetic cycle.
