Red armor penetration square

vincentlondon

What is the red armor penetration square on the toon of the ennemy team sometimes?
Which character is able to create those type of damage ?
I notice this thing seems quite powerful as it makes white negative number which seem to be extra damage

Jabberwocky

Ventress seems to get it quite often when she stuns though it's not listed except in her 5th ability (unique) at level 4.