"Reappearing" antagonists/NPCs are (usually) recorded on 5x7 note cards- name, Characteristics, Movement, Wounds, Skills/Talents, Armour/AP, weapons are recorded on the front, while personality, mannerisms/foibles, preferred hangouts/style of clothing, "Achilles heel" and other "fluffy" bits are recorded on the back.
Cards are often updated. By way of example, the Auditors (Acolytes) had three "face-to-face" encounters with Myrchella Sinderfell, and she was dressed/equipped differently in each encounter. Similarly, her manservant (Warner) was first encountered wearing the stereotypical white glove butler attire, then (in the desert) he dressed like Steve Irwin in a pith helmet.
Even "mooks" and minions get recorded on "throw-away" cards- I like to think everyone has an objective in life, so even these adversaries get a once over through the imagination. As an example here, the Auditors' Inquisitor entered into a contract with a mercenary lodge, the Irascors, wherein she paid for their services as hired guns/muscle and agreed to pay any wergeld to secure their safe return should they be captured- these guys were essentially Goliaths, but they had quite a bit of personality, and the Players actually came to view these "minor" NPCs as full-fledged members of their cell.