How do you write Mery in Japanese?

Answer 1

If "Mery" is a proper name, non-Japanese names are written in Japanese using the phonetic script, which can only approximate English sounds, since many English language sounds do not exist in Japanese.

The word "Mery' doesn't exist in the English language. Maybe checking for spelling mistakes in your question would help.

The English word "Merry" is "陽気な" when written in Japanese script, or "Yōkina" when in European script.

The English word "Mercy" is "慈悲" when written in Japanese script, or "Jihi" when in European script.

Answer 2

Assuming you mean it as a name, you write it like ミレヤ /mi re ya/.