In the Green World, reckless use of biological weapons resulted in plants attempting to swallow up everything, so while accepting their extinction, humanity decided to try and coexist with nature. The side effect of the drugs they used to adapt to the environment is plants growing from their body, such as horn-like branches growing from their head.
Hourai are quite knowledgeable in the use of blades and fire, which they need to carve out living quarters. This talent and training has resulted in their unique form of sword fighting, based and refined on fighting the wild overgrowth of plants. On the other hand, they have adapted their way of life using the plants to their advantage, using it for food, clothing and housing.
If you stumble across their villages, which abandoned the quirks of the industrial revolutions, you would feel like you stepped into a present day costume drama, as they show a pronounced Japanese influence.