Zenitsu does not leave much of an impression when he first meets Tanjiro during the final selection ceremony, as the young blonde seems to have little desire for battling demonic forces. Zenitsu reveals that he was pressured into joining the Demon Slayer Corps by his mentor and idol, Jigoro Kuwajima. Despite his reservations about fighting monsters, Zenitsu makes use of his impeccable swordsmanship and manages to make it through the obstacles on Mount Fujikasane, passing the test and joining Tanjiro in the ranks of the Demon Slayer Corps. Those who reach the final selection are gifted a messenger bird (usually a crow) which forms an integral part of communication between the hunters and the Corps. These birds ensure that the demon hunters always know where they are needed next, however, the feathered creature assigned to Zenitsu was not a crow but a cute and cuddle sparrow.
Chuntarothe the sparrow is an eloquent fit for Zenitsu for several reasons. When looking at the symbolism in Japanese culture, sparrows represent 'the soft demeanor of deep thinkers' and are often considered to bring good fortune- and Zenitsu's name literally includes the kanji for good! As the timid one of the group, Zenitsu is more prone to avoid conflict and does not have much faith in his own skills - having only perfected one Thunder Breathing Technique (so far). Out of all the boys, he definitely has the most gentle nature.
Many fans believe that Chuntarothe directly references the Japanese legend of Shita-Kiri Suzume (the Sparrow Fairy), which tells the tale of a kind-hearted farmer and his unsympathetic wife. After finding a sparrow struggling for its life, the farmer takes the small bird home and attempts to recuperate it, much to his wife's disgust - she sees no value in aiding the helpless creature, even going so far as to cut off its tongue.
Eventually, the sparrow is reunited with its family and offers the farmer a reward, giving him a choice between a small and large chest. Humbled, the man takes the petite option, gratefully finding treasures inside. Curiosity got the better of his wife, who tacked down the sparrow and demanded the large treasure chest as payment - only to discover snakes and dangerous monstrosities within.
The size of the sparrow and the chosen chest are directly proportional to the size of Zenitzu's ego - and to have a small ego leaves much room for personal growth and little room for greed. A power-hungry demon slayer could end up causing more harm than good, and without a selfless nature, they will begin to lose sight of their true purpose - to protect the weak.
Apart from the symbolism, Chuntarothe also reflects Zenitsu's personality. The sparrow is small and seemingly helpless much like his owner, who comes off as the least powerful of the three young demon hunters. His constant complaining, crying, and fleeing in fear is quite frustrating for his companions, and yet Zenitsu is equally annoyed when Chuntarothe gets a bee in his bonnet. The bird's little temper tantrums remind viewers of when Zenitsu makes a scene about something silly.
However, an irritated Chuntarothe is another adequate representation of the blondie's personality - despite his size, the sparrow can still get the job done, and regardless of his self-confidence, Zenitsu is still a potent Demon Slayer. Dynamite does, after all, come in small packages.