Jessica is a black-and-white cat whose predominantly white fur features a distinct dark patch encircling her right eye and ear. Her deep black, oval-shaped eyes possess visible eyelashes. She typically wears a turquoise jumpsuit accessorized with a beige sash, golden bracelets matching James's, and yellow loafers; her tail is usually not visible. When her eyes close, the right one becomes hidden and is occasionally omitted from depictions.

She carries an elegant demeanor, generally treating Cony with kindness and friendliness, though she can adopt a strict tone. Jessica frequently notices overlooked details and offers advice to friends facing difficulties. However, she demonstrates a capacity for cruelty, particularly towards male characters, when sufficiently provoked.

Her narrative presence varies significantly across episodes. She appears in numerous stories, ranging from central roles like in "James the Party Animal," "Silent But Deadly," "Dodgeball," "Keep Calm and Yoga On," "Mr. Ice Guy," "Friendsgiving," "Magical Winter Wonderland," "Hardboiled," and "The Brown & Jessica Date," to being absent from others such as "Spaghetti from the Other Side" and "Three’s a Crowd." Despite an early introduction, she predominantly functions as a background character and is noted as the least popular female figure within the group. She is the eldest and tallest female character.

Her relationship with Moon remains ambiguous, portrayed as married in one episode though this status remains inconsistent elsewhere in the series.