In a standard card deck, the King of Hearts is the only king without a moustache.
The unusual design comes from the traditional French-suited playing-card patterns that influenced modern English cards. The four kings represent historical or legendary rulers: Hearts is commonly associated with Charlemagne, while the other kings are linked with figures such as David, Caesar, and Alexander.
The moustache difference is generally explained as an accident of reproduction. As old woodblocks and engraved designs were copied and recopied, small details could wear away or be lost. The King of Hearts' facial hair disappeared from the design, while the moustaches on the other kings remained. Modern decks preserve the asymmetry even though the original reason was probably practical rather than symbolic.
The card is also nicknamed the “Suicide King” because the sword behind the king’s head can look as if he is stabbing himself. That apparent action is another artifact of the compact, stylized artwork rather than an intended story detail.