The Flame Hashira

Rengu (Rengoku)

Rengu stands in an orange autumn clearing across the river, easy to spot by its orange trees, and Nezura is just south of him.

His bar has four lines: the Flame Katana, Flame Haori Style 2, the Flame Slayer Uniform and Purgatory, Flame's last move, now at 10%.

The Flame Hashira Rengu: Flame Haori Style 2 x1 (3%), Flame Katana x1 (5%), Flame Slayer Uniform x1 (5%), and the struck Purgatory line.

The orange clearing over the river

An orange-leaved autumn clearing west across the river from the shrine south of Bamboo Grove, with the Final Selection Plains further west.

Teleport to the shrine south of Bamboo Grove.
Head west over the river.
The ground and trees turn orange; Rengu stands there. A long wooden bridge leads on from him.

One creator rates the Flame Katana the best katana in the game and upgrades it through dungeons. Learning Flame is on the Flame page.

3,000 health alone, 1,000 more for each extra player.
Each kill costs 120 Reputation; Nezura, a demon boss just south, is there if you need to balance it.
A luck boost multiplies every line on the bar and says so, such as (2x Luck), so a boosted session is the time to farm him.

Where is the Flame trainer?: On a roof by the long bridge out of Mistfall; the boss stands on the far side. Both are on the Flame page.

The demon side has its own bosses, starting with Gyutai; every boss is compared on the bosses page, which also has the drop odds calculator.