Tanjiro’s Water Breathing says MIZU NO KOKYU in Japanese.
MIZU = water
NO = of
KOKYU = breathing
Water Breathing is one of the Total Concentration Breathing and has the oldest history among the five breathing styles of the Demon Slayer Corps.
It’s only Water Breathing that has a technique to kill demons without giving them pain and give peaceful death. I think this feature really matches Tanjiro’s merciful personality.
It requires a calm and quiet mind like the water surface to enhance the skills of Water Breathing.
If you want to know about the water kanji symbol, check the post below! You’ll know what the shape symbolizes.