Schrodinger, the ubiquitous catboy genetically engineered by Millennium's Doctor, first appeared at the London joint reunion of Hellsing and Iscariot heads with the Queen of England. Appearing out of nowhere in an instant without triggering any alarms, he presented all present with a TV set through which the Major declared his intention of an eternal war. Schrodinger's head was promptly blown away by a Jackal bullet, as all present tired of the Major's speech. However, his body vanished without a trace equally fast.
He reappeared, unharmed, at the Millennium HQ in the Hindenburg II seconds later, informing the Major of the result of the meeting. The Major, satisfied with the result, sat upon his throne, petting Schrodinger (something both enjoyed). He, as the Millennium Group courier, also was present at the H.M.S. Eagle's deck, indicating the Huntress Rip Van Winkle how proud Millennium was of her and how well she had served, all the while as Alucard proceeded to gruesomely kill her by impaling her upon her own musket. He flitted through the battle with his teleportational powers and was also seen, chastising Millennium lieutenant Zorin Blitz for disobeying HQ's orders to stand ground until reinforcements were ready to storm Hellsing HQ, within her own mind/illusory world. He witnessed the devastation of London perched upon the spires of St. Paul's Cathedral.
Schrodinger's powers stem from his belief, which, depending of what he believes in, either allows for extreme teleportation or regeneration; if he believes he is somewhere of that he is unscathed, then he is. This, of course, along with his name, is a reference to the Schrodinger's Cat axiom in physics. He has a form or rivalry for the Major's attention with his creator.
He later decapitated himself with a SS knife, allowing his blood to seep in the torrents of blood Alucard was draining, thus becoming the poison the Major envisioned. As he existed everywhere and nowhere as long as he could recognize himself, he was utterly and completely lost within the millions of souls stored within Alucard; thus he existed neither anywhere nor nowhere, nor did Alucard himself, as he had absorbed the soul of Schrodinger. This causes Alucard to vanish slowly, leaving only the Hellsing pentagram inscribed in blood upon the place where he left. Thirty years later, however, Alucard managed to return by destroying all souls within him except for Schrodinger's-returning and acquiring his full abilities in a single stroke.