Is Tanjiro a Demon King?

In the finale of the Demon Slayer manga series, Tanjiro becomes the new Demon King when Muzan put his soul into Tanjiro's body. However, things take an exciting turn when with the help of Tamayo's medicines and Nezuko's calling, Tanjiro fights with Muzan for control over his own body.

Is Tanjiro the strongest demon king?

To add on to this you can see that Tanjiro is also the strongest Demon Slayer of his time because he can now use Sun Breathing PERFECTLY after learning from his visions.

Why is Tanjiro called demon king?

Tanjiro can be considered as the new Demon King when he transformed into a demon. The reason behind this was because the former Demon King has finally been defeated, and there is no other demon left after the Final Battle.