  We present a study of optical and near-infrared (NIR) spectra along with the
light curves of SN 2013ai. These data range from discovery until 380 days after
explosion. SN 2013ai is a fast declining type II supernova (SN II) with an
unusually long rise time; $18.9\pm2.7$d in $V$ band and a bright $V$ band peak
absolute magnitude of $-18.7\pm0.06$ mag. The spectra are dominated by hydrogen
features in the optical and NIR. The spectral features of SN 2013ai are unique
in their expansion velocities, which when compared to large samples of SNe II
are more than 1,000 kms faster at 50 days past explosion. In addition, the long
rise time of the light curve more closely resembles SNe IIb rather than SNe II.
If SN 2013ai is coeval with a nearby compact cluster we infer a progenitor ZAMS
mass of $\sim$17 M$_\odot$. After performing light curve modeling we find that
SN 2013ai could be the result of the explosion of a star with little hydrogen
mass, a large amount of synthesized $^{56}$Ni, 0.3-0.4 M$_\odot$, and an
explosion energy of $2.5-3.0\times10^{51}$ ergs. The density structure and
expansion velocities of SN 2013ai are similar to that of the prototypical SN
IIb, SN 1993J. However, SN 2013ai shows no strong helium features in the
optical, likely due to the presence of a dense core that prevents the majority
of $\gamma$-rays from escaping to excite helium. Our analysis suggests that SN
2013ai could be a link between SNe II and stripped envelope SNe.
