Professional Pet Zombie
4 cost, 3/4
When a Professional damages the Plant Hero, this gets +1 strength
Professional Pirate Zombie
5 cost, 5/3
When this hurts the Plant Hero, steal a section of their block meter
Zombies here get +1 strength and do double damage
Name: Cosmic Pirate
Tribe: Pirate Zombie
Ability: When played: Conjure a Pirate, and it gets Amphibious.
Set - Rarity: Fandom - Rare
"If we Pirates rule the seas, why don't we actually go in them?"
Name: Super Swimming Freak
Tribes: Ultimate Sports Zombie
Abilities: When another Amphibious Zombie is hurt, all Amphibious Zombies get +1 Health. When played: A Zombie gets Amphibious and moves to the Water.
Try as they might, the Zombies couldn't convince her that donut-stroke is a fictional stroke.
Only Master Thief might require a decent rarity to balance, although even then I'd say not really. The rest are fine at just about any rarity. Plus, the abilities of a card should allow one to intuit roughly how hight their rarity is, if that really matters.
Too situational. Amphibious isn't that strong of a Trait, and one shouldn't increase a card's cost by much because of synergy (which I see you're clearly making) unless it was already borderline.
Seems... decently balanced. Not very Sneaky though.