You'd think the Zombie Heroes would actually be called villains, since they work for Zomboss, and yet when you play as them, the plants are the villains. Remember that a hero is someone who is "admired or idealized for courage, outstanding achievements, or noble qualities." It's not a battle of Good vs. Evil, it's a battle of "Plant ideals" vs. "Zombie Ideals".
The Plant and Zombie classes make more sense when you take a closer look at the characters' appearances and personalities:
Guardian: Citron (a Plant Hero who is able to summon a shield to protect himself), Grass Knuckles (Bonk Choy, the plant he is based on, defends other plants by punching zombies), Spudow (Potato Mines are usually used as defense against the first zombie in the first game and sequel), Wall-Knight (who is able to activate a full body of armor), and Beta-Carrotina (a member of the Plant-etary Space Force, which protects space from danger).
Kabloom: Nightcap (whose signature superpower, while not causing an actual explosion, is named Mush-Boom), Captain Combustible (a fire-based Plant Hero with an explosive temper), Solar Flare (a Plant Hero who has fire powers), and Spudow (Potato Mines are explosives that will One-Hit Kill most zombies that touch them).
Mega-Grow: Green Shadow (a leafy Plant Hero who fights with peas), Captain Combustible (a Plant Hero who is able to strengthen Peashooter-based plants), Grass Knuckles (a leafy Plant Hero who packs a punch), and Chompzilla (a powerful superpowered Chomper with shades of green on her body).
Smarty: Citron (a Plant Hero from the future who fights with technology), Nightcap (a stealthy ninja mushroom), Green Shadow (an agile Plant Hero with Super-Reflexes), Rose (a strategic sorceress who can transform zombies into goats), and Beta-Carrotina (a member of the Plant-etary Guard who uses a jetpack).
Solar: Solar Flare (a Sunflower-based Plant Hero), Wall-Knight (his armor has a Flower Motif if you look harder), Chompzilla (a massive Chomper with flower-like leaves on the back of her head and has a strange light green glowing light at the back of her head), and Rose (another flower-based Plant Hero).
Beastly: Brain Freeze (a superpowered Zombie Yeti), Electric Boogaloo (a Zombie Hero who has some destructive and electrifying dance moves), the Smash (a destructive wrestling Zombie Hero), and Immorticia (An undead witch who is able to summon Zom-Bats).
Brainy: Super Brainz (he has "Brainz" in his name, he has a brain medallion on his cape, and he once used a mirror to reflect Rose's wand to turn her into a goat), Rustbolt (a Zombie Hero with a body of junkyard armor that comes with a giant shrink ray), Professor Brainstorm (a Mad Scientist-type Zombie Hero whom Dr. Zomboss envies), Immorticia (a Zombie Hero who manages a counterattack from the Plant Heroes), and Huge-Giganticus (a small but rather intelligent superpowered Imp who plans to launch Meteor Z onto Earth).
Crazy: Electric Boogaloo (a Zombie Hero with erratic and deadly dance moves), Impfinity (a mischievous superpowered Imp who is quite a troublemaker with his cloning powers), Professor Brainstorm (once again, he's a Mad Scientist), and Z-Mech (a reckless Imp who pilots a giant mech).
Hearty: Rustbolt (a Zombie Hero who has orange armor), the Smash (a superpowered version of the notoriously strong Gargantuar with shades of yellow on his outfit), Neptuna (an undead mermaid Zombie Hero who can heal zombies and has an orange fish tail), and Z-Mech (an Imp who is protected by his orange robot mech).
Sneaky: Brain Freeze (a superpowered Yeti Zombie, who, in the first two games, are rare zombies only found in certain levels), Super Brainz (an elusive Zombie Hero who can fly and also tries Clark Kenting after his defeat in his mission), Impfinity (a mischievous Imp Zombie Hero who loves to cause trouble and is quite troublesome when he clones himself), Neptuna (a strange undead zombie mermaid who lives in the ocean), and Huge-Giganticus (a mysterious Zombie Hero from space who plots to crash Meteor Z onto Earth).
The Red Alert Limited-Time Mission (available as a promotion to the RED+ Charity) seems to have nothing to do with AIDS until you realize that zombies themselves are a walking, shambling virus and the Geranium plant (which is also conveniently a flower, much like Rose and most of the plants she uses) is used to Cure HIV-1.
During gameplay, Plant teammates face to the right while Zombie teammates face to the left, regardless of which side the player is playing. Now, in Plants vs. Zombies and its sequel, which direction did the plants and zombies (generally) face during gameplay?
The turn order mirrors the gameplay in both the original game and its sequel. Zombies play their units first (Zombies enter the lawn), Plants play both units and tricks (like placing a Peashooter or using a Iceberg Lettuce for example), then Zombies use their tricks (such as Gargantuar Prime, Wizard Zombie and Octo Zombie using their abilities on the plants you've placed down) before both sides attack.
Primeval Yeti's original description states that his lunchbox is just a solid block because he hasn't invented "opening things" yet. Yet, his Evolution ability clearly has him open his lunchbox to give the other zombies a buff. At first, this looks like blatant Gameplay and Story Segregation, but since he only does it when Evolved from another zombie, it allows him to do so because he evolves the method to open it!
The zombie mission "Everything’s Coming Up Goats" ends with Rose hitting herself with the Goatify spell (thanks to Super Brainz’ mirror), this turning her into a goat. This means Rose has the magic to turn zombies AND plants (and presumably humans as well) into goats. Now, imagine if a zombie were to get their hands on her wand...
The zombies would need to know how the spell works, as Rose is a powerful sorceress and knows how to cast a Forced Transformation spell. And they probably do — Immorticia is the Zombies' resident sorceress and might be able to know how to cast it, much like the Wizard Zombies from the second game.
