Throughout Demon Slayer's story, Tanjiro has gained some pretty incredible abilities and powers. He learned the ten forms of Water Breathing from the former Water Hashira, Urokodaki. He awakened his Demon Slayer Mark while fighting the Sixth Upper Moon, Gyutaro. And he's learned Sun Breathing, used by the strongest Demon Slayer to ever exist. All these abilities are great, but none compare to the ability he was simply born with: his sense of smell.
Tanjiro's sharp sense of smell has always been his strongest, albeit strangest, ability. His nose has saved his life and given him an advantage over his enemies more than any of his Demon Slayer training. The level of perception that Tanjiro has achieved through his sense of smell alone goes far beyond human limits. Although Tanjiro's nose is likely his weirdest characteristic, it fits the theme of his character perfectly.
Tanjiro’s Nose Can Smell the Future
Tanjiro's sense of smell has allowed him to perform some truly amazing feats. In the very first episode, he was able to smell the scent of a cat on a broken vase. He could track a person from the top of a mountain all the way to the bottom just from their scent. Tanjiro can even detect the smell of demons from far away and tell how strong they are based on their decaying scent. He was able to recognize Muzan's scent years after he killed his family despite having never directly met him before.
Strangely enough, Tanjiro can even detect things with his nose that don't produce a smell. For instance, he can smell a person's true intentions and emotional state. He could smell Shinobu's anger toward demons despite her masking her emotions with a cheerful demeanor. Most impressively, Tanjiro can smell the pause in an enemy's defense, letting him know when there's an opening to strike. He visualizes this smell as the opening thread.
However, Tanjiro's most unbelievable feat is his ability to smell the future. During the "Swordsmith Village" arc, Tanjiro trained against a six-armed battle doll called Yoriichi Type Zero, which was modeled after Yoriichi himself. While fighting the doll, Tanjiro unlocked the ability to smell the future to predict the doll's next six moves. Tanjiro's sense of smell has given him a form of precognition.
No-One Can Match Tanjiro’s Nose
Unlike all of Tanjiro's other abilities and skills, his sense of smell didn't come from someone else. His training in Water Breathing came from the former Water Hashira. His knowledge and proficiency in Sun Breathing originated from Yoriichi Tsugikuni and were passed down through his family with the Hinokami Kagura dance. His Demon Slayer Mark came from his close connection to Sun Breathing. Even his incredibly hard head that can stun demons was inherited from his mother.
In all of these things, Tanjiro isn't even the best; he is out-skilled by more experienced fighters or older masters of their respective skills. Tanjiro's sense of smell is the one ability that he didn't learn or inherit from someone else. It's also an ability that no one else has. Even demons that have superhuman senses and supernatural powers lack the accuracy of Tanjiro's nose.
A heightened level of smell is the perfect ability for a character like Tanjiro, who is one of the most pure-hearted characters in all of anime. The core of his soul manifests as a beautiful clear blue sky. He doesn't have ulterior motives and isn't biased by prejudice. It makes sense that Tanjiro would have an ability that allows him to perceive a person's true self.
Tanjiro’s Nose Is His Greatest Power
Tanjiro's sense of smell may not be as flashy or powerful as Sun Breathing or his Demon Slayer Mark, but it's easily his greatest. One of the biggest disadvantages to Tanjiro's Breathing techniques and his Demon Slayer Mark is the drain on his stamina. Tanjiro can only maintain these abilities for a short amount of time before he is completely drained. Conversely, his sense of smell doesn't affect his stamina or cause any other drawbacks.
Tanjiro's ability to perceive the opening thread has been the deciding factor in the majority of his fights. Without this ability, he would have fallen prey to one of the many demons he has faced. Being able to detect the opening thread was the only reason Sakonji even allowed him to take part in Final Selection and become a Demon Slayer. His extraordinary sense of smell is his most-used skill.
Because Tanjiro can smell a person's true intentions, he always puts his trust in the right people. Despite Zenitsu's cowardice and Inosuke's aggressiveness, Tanjiro knew they were good people at heart. His kindness is never taken advantage of because he knows what people are attempting to use him. As a result, Tanjiro's sense of smell has allowed him to make trustworthy allies and detect all kinds of danger.