He is mentioned in one of the Call of the Dead radios by Edward Richtofen who states that he might have killed "the specimen from Mexico" and that his spleen is on the floor. In one of Richtofen's letters leading up to Origins, he says Pablo's journal had been the most valuable source of information for finding Element 115. In Der Eisendrache, there's a projector which displays some images, one of them being a spleen on a tray which is labeled "Pablo". A letter from him can be found as a cipher in Gorod Krovi. He is mentioned by Ultimis Tank Dempsey in Classified.
"My name is Pablo, or at least I think it is or was, I don't know anymore. I have been locked away, held prisoner in this cell for many months by German scientists. They keep experimenting on me, pumping me full of some blue liquid. I think it is responsible for the repeated visions I have been having, which I swear look and feel real. In my vision, my name is Pablo Marinus and I am a knight coming back from the Crusades. I fight in a great battle against strange, demon like creatures who are trying to devour the Earth. At one point, it appears I am doomed, trapped in the tentacles of a great, three headed beast, when suddenly, four knights decimate it with magical elemental staves, saving me from certain death. There are other worldly creatures that fight on our side against the demons. The creepy thing is that the sigils on their tunics resemble the ones I noticed on the stones here at the castle as I was being brought to my cell. I just hope I can eventually find a way to escape this place."
— Pablo describing his experiences as a prisoner and the visions he had when tested with Element 115.