The behavior of superconducting vortices near a twin boundary (TB), a common planar defect in YBa₂Cu₃O_7-x (YBCO), has bearing on diverse issues in statistical mechanics such as the kinetic roughening of driven interfaces. We report measurements of individual vortices in an ortho-II YBCO single crystal with unidirectional TBs with 3-6 µm separation. Using low temperature magnetic force microscopy (MFM) with single vortex resolution we study the behavior of vortices in the vicinity of a TB by imaging and by dragging individual vortices with the MFM’s magnetic tip at temperatures down to 5K. Our measurements reveal the anisotropic behavior of individual vortices on TBs – it is easier to drag a vortex along a TB than across it. This is manifested by a characteristic deformation of the regular shape of an unperturbed vortex as it is dragged near a TB. Surprisingly, we find a significantly higher resistance to dragging a vortex along one of the directions perpendicular to the TB than along the opposite direction. When exerting sufficient force on vortices in regions between TBs, our measurements reveal the pancake nature of vortices in underdoped YBCO. This is manifested by vortices breaking into partial stacks of pancakes. Preliminary studies show that we can probe the long time scales associated with the attraction of the partial stacks that constitute a full vortex. Finally, we use the MFM tip’s repulsion from the sample due to the Meissner effect to search for variations in the superfluid density near TBs inspired by recent work on the pnictides [Kalisky et al., Phys. Rev. B81, 184513 (2010); Kirtley et al., Phys. Rev. B81, 184514 (2010)].