  We present results from the Hitomi X-ray observation of a young
composite-type supernova remnant (SNR) G21.5$-$0.9, whose emission is dominated
by the pulsar wind nebula (PWN) contribution. The X-ray spectra in the 0.8-80
keV range obtained with the Soft X-ray Spectrometer (SXS), Soft X-ray Imager
(SXI) and Hard X-ray Imager (HXI) show a significant break in the continuum as
previously found with the NuSTAR observation. After taking into account all
known emissions from the SNR other than the PWN itself, we find that the Hitomi
spectra can be fitted with a broken power law with photon indices of
$\Gamma_1=1.74\pm0.02$ and $\Gamma_2=2.14\pm0.01$ below and above the break at
$7.1\pm0.3$ keV, which is significantly lower than the NuSTAR result ($\sim9.0$
keV). The spectral break cannot be reproduced by time-dependent particle
injection one-zone spectral energy distribution models, which strongly
indicates that a more complex emission model is needed, as suggested by recent
theoretical models. We also search for narrow emission or absorption lines with
the SXS, and perform a timing analysis of PSR J1833$-$1034 with the HXI and
SGD. No significant pulsation is found from the pulsar. However, unexpectedly,
narrow absorption line features are detected in the SXS data at 4.2345 keV and
9.296 keV with a significance of 3.65 $\sigma$. While the origin of these
features is not understood, their mere detection opens up a new field of
research and was only possible with the high resolution, sensitivity and
ability to measure extended sources provided by an X-ray microcalorimeter.
