Jan Valentine was an arrogant, chauvinistic, foul-mouthed and none-the-too bright Millennium grunt, a subordinate of his older brother Luke. While the suit-clad Luke had upper-class charm, Jan dressed like a stereotypical chav and was constantly spouting profanity as well as crass and volatile statements, much to the annoyance of his snobby brother. Both brothers had submitted to the Vampirization process, which left Luke endowed with amazing speed and granted him access into the elite Werewolf circle, even if only as a warrant officer on par with Schrodinger, while Jan demonstrated no such abilities outside of being a baseline Millennium vampire. He, as the more brash and reckless of the brothers, was entrusted with a small ghoul army by his brother to lead against the human servants of the manor and the members of the Convention of Twelve.
With his ghoul army wearing body armor and carrying riot shields (with "Boo Doo People Murder People" scrawled on the front accompanied by the Egyptian-style eye symbol that appears on Jan's hat) Jan wielded a custom pair of suppressed P90 sub-machine guns that he dual wielded with great skill. At point, Jan shoots and kills multiple Hellsing soldiers in a four direction hallway while reciting the infamous "Konami Code" of "up, up, down, down, left, right, left, right," as a reference to the video game company Konami. He stomped, nearly without true opposition, escorted by his group of armored ghouls, into the Hellsing Manor, brutally slaughtering practically the totality of the Hellsing operatives, until he reached the mansion's third floor and the last corridor, whereupon he was engaged by Hellsing trashman Walter C. Dornez; he made the mistake of insulting the elderly butler, and had a lip piercing ripped out for his troubles. When he attempted to have his ghouls kill the man, Dornez responded by living up to his nickname of "Angel of Death" and flicked out his trademark monofilament wires, razing through Jan's men. Valentine merely responded by laughing maniacally and having an armored phalanx advance upon Dornez.
This would have worked, except for the intervention of the Draculina Seras Victoria. Using her massive weapon, the Harkonnen breech loader cannon, she pierced the ghouls' defensive lines, allowing for an explosive volley to incinerate the remains of the original battalion. Valentine then attempted to assault the butler, but Victoria detained and pinned him down. Upon this, Walter started torturing him by crushing his hands under his heel, extracting the information he needed to stop the attack. Eventually, Valentine distracted Victoria enough to swat her aside, as he ran to reach the Convention of Twelve room, even after losing an arm to the butler's razor-sharp wires. Bursting in, he was greeted by a volley of gunfire.
After the defeat of the Hellsing ghouls by Victoria, Integra Hellsing started prodding him for information herself. He demonstrated some understanding and knowledge of the microchips impanted within all artificial vampires, their purpose, and possibly even understood some of their workings, but, knowing he would die, he gave Hellsing a clue on their enemies: in his final moments before the microchip burned him to death, he muttered a single word: "Millennium".
In the anime series, he is portrayed as a stereotypical thug and not much more, co-owning with his brother a seedy nightclub which he used as an excuse to pick up women, vampirize them, and have them become prostitutes or dancers at his club. While he still pronounces the same final word, the Millennium idea was taken no further.