Optical spectra for nearly 90 knots in Cas A's northeast jet, together with [S II] λλ6716, 6731 and Hα imaging, are used to provide a detailed look at the jet's kinematic and emission-line properties. The jet is a 50" wide, fanlike arrangement of line-emission knots extending up to 3' outside Cas A's main shell. Knot expansion velocities range between 7000 and 13,000 x (d/3 kpc) km-1 with the majority of radial velocities lying between -3000 to +1000 km s-1, implying an orientation within 6° of the plane of the sky and an opening angle of ≃25°. The jet's optical emission consists of three knot types. Most show strong O, S, and Ar line emissions like other fast-moving knots (FMKs) in the remnant, but with comparatively weak [O III] line emission. A few jet FMKs show elongated morphologies suggestive of mass ablation due to Kelvin-Helmholtz instabilities along knot edges. Six high-velocity knots with strong [N n] emission ("fast-moving flocculi," or FMFs) are also found in the jet region. These FMFs lie along the southern edge of the jet and show a velocity range of 7000-9000 (d/3 kpc) km s-1. In addition, about a dozen jet knots are found that exhibit both FMK- and FMF-type emissions, often with comparable line strengths. Such "mixed emission knots" or MEKs have an FMK-like velocity range of 8500-1O,500 (d/3 kpc) km s-1 and form a coherent and kinematically distinct structure. We suggest MEKs represent a debris hybrid caused by turbulent mixing of different progenitor layers. The Cas A jet may be the most visible example of a dozen or so ejecta plumes of core/mantle material in Cas A, possibly related to the high-speed expansion "fingers" seen in recent two-dimensional core-collapse models.