This question is closely related: What is the difference in usage between a plural using (a) the kanji repetition character 々, (b) a plural using -たち, and (c) the singular?
In an RPG, 猫の耳 should almost always be sufficient. Whether it's one ear or a pair of ears does not matter in most cases. But in a very rare situation where "cat's ear" and "cat's ears" must be distinguished as two different types of item in an inventry, you can use 猫の片耳 and 猫の両耳. These are the words you have to remember and use when you want to specify whether something (e.g., a medical trouble) is only about one ear or about both ears. If this is about "a rose" vs "roses", these can be distinsuished like 一輪のバラ and バラの花束. As the linked article points out, there is no one strategy to specify plurarity in Japanese, so you have to get used to several patterns.
Definiteness of the subject of a sentence is specified using が/は (see: What's the difference between wa (は) and ga (が)?). Definiteness of an object can be explicitly specified if you really need to, too. For example, you can say ある猫 ("a certain cat"), その猫 ("the cat"), 例の猫 ("that cat (you know)"), etc.