If this element is unidirectional or bidirectional ? (see the image)
It's clear that current flows in only one direction through the device, so I guess that means that it's "unidirectional" in that sense.

But the odd thing is that in one half of the diagram, the device is absorbing power, while in the other half, it is generating power. There's no real device that has a characteristic curve like the ones shown.

Devices like electrochemical cells, photovoltaic cells, capacitors, inductors, etc. have the ability to both absorb and provide power, but their V-I curves are very different from this.