BJT LT spice DC transfer characteristic
It seems that you're sweeping VAC from -10V to +10V while holding VCC and VEE steady at 5V each. Note that since you're driving the base with a voltage source, R1 and R2 have no effect on the circuit whatsoever.

The first part of your curve (from -10V to a little over -5V) is where the Q1 is cut off completely. There's no current through RC, and no voltage drop across it, so Vc sits at +5V.

The next part (from -5V to -4V) is where Q1 is starting to conduct. As the current through RC increases, the voltage drops, until Q1 is saturated. At this point, Vc is a little bit less than Vb.

From this point on, Q1 is completely saturated, and Vc tracks Vb with a small offset that represents the forward drop of the B-C junction. This can drive Vc even beyond VCC, as you can see.