RS232 is -12 Volts on the low, so there’s actually 24 Volt difference between one state and another.
I don’t believe it’s possible to do RS232 with any one single transistor if the ground is connected to the other device.

The whole compromise to make 5 Volt RS232 ports is just that.
It could be damaging to connect a real RS232 port to a pic without proper clamping if it didn’t have some awesome IO characteristics.