Naruto Uzumaki has a pretty recognizable design. He wears a headband with a leaf symbol, a bright orange jumpsuit with a red spiral on the back, and Zōri sandals. However, his most distinctive feature has to be the mustache-like markings on either side of his face.
As cool as Naruto’s whiskers are, there is a mystery as to how they came to be. No official source explains them, so fans must speculate on the matter. The obvious answer is that this is a unique and cool design, but maybe there is a good explanation in the universe for their existence.
The first and easiest answer to Naruto’s whiskers question is that it is a design choice. Masashi Kishimoto wanted Naruto to look like a fox in human form. That’s what the original Naruto pilot suggests, in which the boy’s true form is a fox. Ebisu also suggests, in the early chapters, that Naruto is the human form of the Nine-Tailed Fox, although later chapters suggest that this is a metaphor at best. Regardless, the initial concept of Naruto being a fox, whether literal or figurative, is easily evident.
As for why mustaches appear as black lines, that’s the result of Naruto’s original medium: the manga. In a format that is mostly drawn in black and white, black lines are the best way to represent what would likely be relatively subtle marks in real life. An alternative would be to shade Naruto’s skin and add white lines to his cheeks, but that would take a lot more effort than necessary.
Whiskers may just be a creative choice, but not all fans are happy with such an answer and look for clues in the narrative. For example, mustaches seem to be a hallmark, but they’re not exclusive to Naruto. They also appear on Ginkaku and Kinkaku, who have acquired part of the Nine Tail Chakra by eating their flesh. This could mean that the brands are exclusive to those with a connection to Kurama.
This theory about whiskers is reinforced by how the appearance of whiskers changes when the Nine Tail Chakra is used. When Naruto first uses it, his whiskers cover more of his cheeks and become more pronounced. When he reaches higher levels of his Nine-Tailed Chakra Mode, explicit markings appear on his face above his whiskers. These changes seem to indicate that the whiskers are an influence of the Nine Tails.
While Kurama is likely to give people mustaches, there are a few points for debate in this theory. The first is that Naruto was apparently born with his whiskers even before the Fox was implanted in him. Second, Naruto’s mother Kushina was once a nine-tailed jinchuriki, but her face is devoid of markings. Additionally, Naruto’s children have mustaches on their faces while they have never received chakra directly from Kurama. This could all be seen as oversights on Kishimoto’s part, but if not, there must be some other reason for the mustaches.
It’s possible that Naruto’s whiskers are some kind of birthmark. This helps cover up the fact that he had his whiskers before he was imbued with Kurama’s chakra. Granted, the shape and location would be a bit artificial, but birthmarks come in enough shapes, sizes, and even colors for this theory to have some leeway.
Another possibility is that the marks appear in the womb of a person with the Kurama chakra. This allows Naruto and his children to have mustaches. Hinata is not a jinchuriki, but she was imbued with Fox chakra during the Fourth Great Ninja War, so some residual chakra may have influenced Boruto and Himawari. However, that still doesn’t explain the existence of Kinkaku and Ginkaku, which is also questionable.
Whatever the reason, the point is, Naruto’s whiskers have no specific reason to exist. Neither the manga nor Kishimoto explained how they got there. It’s up to the fans to draw their own conclusions on the matter.