Marco is the hospital's pharmacist. He is played by actor Marco Pelaez. He has appeared in five episodes, always in the hospital's pharmacy next to the clinic.
In Detox, House gets mad at him when the hospital's supply of Vicodin has been mis-shipped.
We learn his first name in Whac-A-Mole when Wilson screams at him for not authorising his patients' prescriptions - it emerges that his DEA number has been suspended as part of Tritter's investigation.
In Merry Little Christmas, he has to suffer House's wrath of being cut off a floating prescription.