A Cat's Eye is an obtainable item which allows you to further upgrade your units, except Normal Cats and certain special units such as Bahamut Cat and Valkyrie Cat, past Lv.30. Each Cat's Eye allows the user to increase the maximum level cap of a unit by 1, but the player still needs to use XP to upgrade the Cat to that level. The maximum cap level allowed by using Cat's Eyes is level limit of +10.
Example: Jizo (Lv.30+0) can reach Jizo (Lv.31+0), with maximum cap of Jizo (Lv40+0)
The cat unit also has to reach max level 30 from XP upgrade first. Therefore, theoretically it is impossible for a player to use Cat's Eye on every units before reaching specific User Rank levels. Even if a player gained Cat's Eye from expedition or other ways.
Obtaining Cat's Eyes
Cat's Eyes can be obtained from Gamatoto Expedition with rare drop rates or as special rewards. You can find the number of Cat's Eyes you currently have in the cat upgrade screen.