I think ZOE1 and ZOE2 might be the most drastic leap in quality between sequels that I have ever seen.
The first one feels like a bad Gundam anime rip-off that got a quick game adaptation. The second one is like a super-robot simulator.
(the closest thing to a super robot simulator is Robot Alchemic Drive/RAD, another underappreciated PS2 game)
Actually, let me recommend RAD, now that I remember it. It's a loving parody of 70s mecha anime, where each part of the robot's body is controlled with a different part of the controller; you also have to run your main character around the fighting robots/city battleground (as the mecha is remote controlled like Giant Robo or Giagantor).
Like the EDF games, it has an impressive sense of scale for a PS2 game, but a bit of a shoddy frame rate.
You had three giant robots and three versions of the same protagonist (typical spiky haired mecha protagonist, cool older protagonist, and mecha nerd girl protagonist who protests that liking mecha does not qualify her in any way to pilot one.)
The best robot is the most experimental one, because that one has a super-powered mode with a dramatic, voice-acted countdown until the engine is overloaded. It can dive kick through skyscrapers, and if that doesn't sell you the game, nothing will.
Changing the protagonist doesn't change the story significantly, but changes a fair bit of dialogue (mainly, your own character's.)
Now that I remember where I am posting (it's been a while, I need to play again,) there was a thing if you picked the female protagonist.
Your character has a klutzy best friend with a tragic past (of course,) and her story changes the least
(looked it up; Nanao was the friend, Yui is the name of the third version of the protagonist.)
So, if you picked Yui, some of the dialogue is changed to be about close friendship rather than romance. However, if you focused on her side-story, Nanao still ends up talking about how much she loves the protagonist and you'll end up with her rather than the male love interest. It's a friendship that is straight as a rainbow.