  We present 888 visual-wavelength spectra of 122 nearby type II supernovae
(SNe II) obtained between 1986 and 2009, and ranging between 3 and 363 days
post explosion. In this first paper, we outline our observations and data
reduction techniques, together with a characterization based on the spectral
diversity of SNe~II. A statistical analysis of the spectral matching technique
is discussed as an alternative to non-detection constraints for estimating SN
explosion epochs. The time evolution of spectral lines is presented and
analysed in terms of how this differs for SNe of different photometric,
spectral, and environmental properties: velocities, pseudo-equivalent widths,
decline rates, magnitudes, time durations, and environment metallicity. Our
sample displays a large range in ejecta expansion velocities, from $\sim9600$
to $\sim1500$ km s$^{-1}$ at 50 days post explosion with a median H$_{\alpha}$
value of 7300 km s$^{-1}$. This is most likely explained through differing
explosion energies. Significant diversity is also observed in the absolute
strength of spectral lines, characterised through their pseudo-equivalent
widths. This implies significant diversity in both temperature evolution
(linked to progenitor radius) and progenitor metallicity between different
SNe~II. Around 60\% of our sample show an extra absorption component on the
blue side of the H$_{\alpha}$ P-Cygni profile ("Cachito" feature) between 7 and
120 days since explosion. Studying the nature of Cachito, we conclude that
these features at early times (before $\sim35$ days) are associated with
\ion{Si}{2} $\lambda6355$, while past the middle of the plateau phase they are
related to high velocity (HV) features of hydrogen lines.
