Tier

Tiers are a function of a pet's Speed. Each time Speed reaches certain break points, the Tiers available to the Pet increase permanently by one. All pets start off at Tier 1, and can advance as far as Tier 4. Each Tier (1-4) gives the pet one action point per battle turn. Skills are also rated by tier, and their rating directly correlates to how many action points a pet must spend to use that Skill. Since a Tier 1 pet has only one point, it can only use a single Tier 1 attack each turn. A Tier 2 pet has two points, and can use either two Tier 1 skills, or one Tier 2 skill each round. Reaching Tier 2 effectively doubles the amount of damage you can do each turn. Unlocking as many Tiers as possible early on is important for a battle pet.

Increasing a pet's Tier also makes more attacks available at the Skill Shop, since it will only allow you to view and purchase Skills that your pet can actually use (and match its current Type.) It is worth noting that you cannot use Equipment boosts to achieve higher tiers, only the base Speed is factored in.