Shoyu is most properly represented in its Kanji form, of which it has two.

醤油「しょうゆ」 - The most common kanji representation.

It has a second form, 正油, but is used infrequently.

A bit about the kanji:

醤 - Shou - a kind of miso.

油 - yu - oil, or fat.

The second form of Shoyu has only a single character difference:

正 - Shou - Represents correct, or justice. This is a very irregular use of kanji, which is why this representation is not followed often.