Tanjiro's scar has subtly evolved throughout the series without any concrete explanations attributed to it. Tanjiro's scar is a major plot point in the story that is gradually being unveiled over the course of the manga. The mark has raised questions regarding its accessibility, its power and its contribution to the storyline.
1. Quick Answer
In the anime, Tanjiro's scar takes on 3 noticeable forms while in the manga, it takes on a fourth form too. By tracing the trajectory of the scar's growth, it can be determined that the scar evolves once the user unlocks a certain power or upgrades it by growing more powerful.
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It can also be fairly theorised that the scar is a birthmark of the Breath of the Sun Users. This is because of Shinjuro Rengoku's belief that all Breath of the Sun Users possessed the red scar on their forehead just like Tanjiro.
2. Different Forms
Initially, Tanjiro's scar looks like a patch which later develops into the red flame-like patch that spreads across his forehead. The scar seemed devoid of any mystical background as Tanjiro claimed that it was the result of an injury suffered while saving his brother from a hot iron. Yet, the scar is not what it is claimed to be. The 4 distinguishable forms of the scar can be observed in the following events:
- At the beginning of the story, Tanjiro's scar is a frayed patch of skin.
- This skin takes on a jagged appearance in the form of a moon on his forehead after his fight with the Hand Demon.
- It is during his fight with Rui that the evolution of the scar becomes apparent. The scar appears to look like a red flame.
- Finally, in the manga, Tanjiro awakens his Demon Slayer Mark during his fight against Daki. This scar further extends across his face.
With the information at hand, there are several factors that can be attributed to this phenomenon :
3. Awakening of Power
By tracing the trajectory of the scar's growth, it can be determined that the scar evolves once the user unlocks a certain power or upgrades it by growing more powerful. Tanjiro unlocks the second form while using the Breath of the Water and the third form while using the Dance of the Fire God.
3. Birthmark of the Sun Users
Shinjuro Rengoku believed Tanjiro to be a Breath of the Sun User after noticing his scar. It can be fairly theorised that the scar is a birthmark of the Breath of the Sun Users.
Although Tanjiro rebukes this by claiming that the scar appeared after he saved his brother from a hot kettle. But this alludes to the nature of the birthmark - whether it's a dormant mark that activates itself upon acts of compassion. Tanjiro even claims that his father had a slight birthmark too. Therefore, this fact cannot be dismissed as a coincidence or trivial. In Chapter 81 of the manga, when Tanjiro confronts Daki and asks her why people take lives or trample on them. She is reminded of Yoriichi Tsugikuni - the first Breath of the Sun User - although she is unable to place his name. These hints from the author deserve some weight.
Later in the manga, Muichiro and Mitsuri start developing similar marks on their faces while facing against the Upper Moon Demons. The state of Muichiro's body when he developed the scar provides a great insight as to its aspects. Muichiro claimed that he had pushed his body to a limit that his heart rate had increased exponentially and that his body temperature had soared way beyond normal.
Amane Ubuyashiki's input on the marks on Muichiro and Mitsuri's faces expanded the knowledge of the marks and their place in history. It was revealed that the demon slayers who were one step away from doing Muzan away were linked by one aspect: a scar. She further explained that the scar was developed by demon slayers by unlocking their potential or at a certain peak level. It can be surmised that Tanjiro's scar resonates with the other slayers and gives them access to develop such a scar.
6. What does it signify?
The scar is a symbolism of a Demon Slayer that has reached its peak potential in administering their powers. Tanjiro's scar is slightly different from the ones developed by other slayers as it is an original mark (probably utilised by the Breath of the Sun Users) that is able to unlock a certain path for the other slayers.