Regen is an effect that has every character when they are in EX Mode.
While in EX Mode, regen slowly fills in a small amount of HP.
With the help of accessories, you can higher the percentage for recovery HP regen.
When you are in EX Mode for a very long time, with the help of accessories to lenghten your EX Mode duration, you can recover more HP in the meantime.
When the EX Gauge goes empty and you will transform back normal, the Regen effect doesnt apply anymore and will no longer recover HP unless you turn to EX Mode again