Cross was in charge of the medical team that were to give Queen Perdita of Vlatava a new heart, which had to be delivered by Kid Flash. He waited anxiously, and when he finally found Kid Flash, it turned out someone else had already claimed it. Kid Flash managed to retrieve it, and handed it to Doctor Cross. The surgery was a success.
Doctor Cross helped Ray Palmer with an operation on Blue Beetle. Cross stood by while Palmer and his protégé, Bumblebee, explored Jaime from the inside, trying to remove the scarab. When the scarab's defenses made it impossible to operate it, Atom contacted Cross for extraction. He inserted a syringe, giving them a means of escape, and emptied it on the surgery room floor.
In the comics, Pieter Cross is a Norwegian-born blind doctor, and the second Doctor Mid-Nite.
Though Doctor Mid-Nite has appeared in Justice League Unlimited, Justice League: The New Frontier and Batman: The Brave and the Bold, those were all Charles McNider, the first Doctor Mid-Nite. This is Pieter's first animated appearance.