Upon surviving the week of final selection, applicants to the Demon Slayer Corps are measured for uniforms, allowed to choose ore for their swords and given a very gifted companion -- a bird that will serve as a messenger, informing them of their next missions. However, line these birds up and there's an obvious outlier among the lines of glossy crow feathers: Chuntaro the sparrow, who serves as the messenger bird for Agatsuma Zenitsu. But why does Zenitsu receive a sparrow companion? The answer lies in old Japanese folklore.
The use of the messenger crow is well-known in Japanese mythology. According to a certain legend, a three-legged crow was sent from the heavens to the hero Jimmu from the goddess Amaterasu. The three legs represent characteristics of the gods: wisdom, kindness and courage. This crow -- the Yatagarasu -- flew to Jimmu and instructed him where to go to battle, where he emerged victorious. In Demon Slayer, the use of messenger crows that tell the members of the Demon Corps where to battle pays tribute to the Yatagarasu. The crow in general is also meant to be a symbol of protection (not to mention having a stellar sense of direction).
The sparrow, as opposed to the crow, has a less auspicious legacy, but that's exactly why Zenitsu and Chuntaro are perfect for each other, as Zenitsu does not represent the crow at the start and instead will grow into those characteristics. He is introduced as a character who needed to be saved from debt, so he's not necessarily wise. However, he is a kind person, demonstrating this by protecting Nezuko from Inosuke Hashibara when the two first meet.
Nonetheless, he often hides behind those weaker than he is and goes on long rants about the danger they're in. He's certainly is not courageous. However, a Japanese folktale called "The Story of the Cut-Tongue Sparrow" reveals why the sparrow instead was chosen as a fitting companion for Zenitsu as he matures.
In the story, there is an old woodcutter and his greedy wife. One day, the woodcutter finds a hurt sparrow and feeds it rice, much to his wife's consternation (and very fitting given how Zenitsu feeds Chuntaro). On a day the husband is away, the sparrow eats some starch when the wife doesn't feed it, so she cuts out its tongue and throws it out of the house.
When the husband returns, he sets out at once to find the sparrow and discovers a whole nest of them, who reunite him with the one he rescued. The sparrows offer the woodcutter two chests, one large and one small. Being humble and aware of his physical limitations, the woodcutter takes the small chest home, only to discover that it's full of treasure. His wife immediately rushes out to find the sparrows and takes the large chest. Impatient to be home, she opens the chest while walking and discovers it to be full of snakes and other horrors, which causes her to startle and topple off a cliff.
While the tale has several morals, the one most pertinent to Zenitsu is the chest of treasures. Zenitsu lacks faith in his abilities, metaphorically choosing the small chest for himself. However, even though he has only mastered one form of Thunder Breathing, his technique is powerful enough to take down quite powerful demons. He is also, as stated before, kind -- a trait that manifests as he grows in confidence and shows especially in the way he sticks up for children and demands the demon Daki apologize for hurting a young girl.
In another way, Zenitsu could be seen as the small chest himself -- something that no one but his teacher saw potential in but found treasure inside. The sparrow in itself is also a symbol of community, kindness and good luck, and Chuntaro's willingness to stick around as a faithful companion may be just what Zenitsu needs as a reminder that he now has support on his journey.
While watching a Demon Slayer get harassed by a tiny sparrow certainly does lend levity to a series that can get quite dark, Zenitsu and Chuntaro are paired for far more than just comic relief. Chuntaro is a symbol of what Zenitsu believes himself to be as well as what he has the potential to become, and that promises great things for the series' resident coward.
