You may show any number of Personality cards in your hand at any time, but if you do you must show them to everyone at the table. You may also ask to see another player’s Personality card(s) — for example, to prove they have a card they promise to play during a later turn — but they may decline if they wish.
No player may be forced to show their hand at any time, except with a Seeker Personality card. This is also the only way to privately see a player’s hand.