"Nature" in sense of 'attribute, tendency, character' is 資質 /shi shi tsu/ and in sense of 'environment' is 自然 /shi zen/. There are other words but these are main and common ones.
Although there are a number of different sites that offer the service of language translation, it has been found that there is not a translation for a Japanese word doujin. There have been multiple efforts to find this translation and the results come up with the same word as a translation. Further research has shown that the word doujin refers to a Japanese cartoon.
kasu
The Japanese translation for gladiator is 剣闘士.
Warlord
"Clarity" is not a Japanese word, but the English to Japanese translation for "clarity" is 明瞭さ (meiryousa).
Mature-
apurikotto
Shizen
the nearest translation/japanese equivilent is 'ningyo'
Cassie isn't a Japanese word, so it has no translation.
"Smith" isn't a Japanese word, so it has no translation. If you're looking for the Japanese word for "blacksmith," it's: kajiya.
To say the word appetizer in Japanese, you would say Zensai. There is a symbolic translation of the word appetizer as well.