These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
In the first game, his goal isn't to be a master or save the world. It's to build up his grandfather's company, which he was put in charge of. He can refuse to help his girlfriend and a kid who are being picked on by bullies. Cody also joins the local evil army and wins at every one of their factions, then defeats their boss. He can sell his Robopon, who are stated to be sentient creatures, for money. At the end he's the most powerful Robopon trainer on Porombo Island, and when he goes to a tournament in the sequel he leaves all of his friends at home.
In the sequel, Nick D. tries to guilt trip Cody into giving him the X-Stones, because he needs them to provide money for the orphan kids at Chapel Academy. You defeat him every time. Cody (accidentally?) floods an entire kingdom with a fishing rod, is sentenced to death by hanging, and escapes without punishment thanks to a time machine. Dr. Don and Sam continually berate Cody for wrecking their time machines. Eventually, it drives Don insane. Then Cody arrives on the bad guys' Battleship 20 years in the past. He does not prevent it from blowing up an entire town, but does blow it up with the villains inside, nearly killing Zeke and causing Zeke's father to berate him. At the end, it's revealed that Cody defeating the villains at the end of the game caused their actions in both games thanks to time travel.
Fridge Brilliance: After unlocking rematches with the Legends in the first game, Dr. Zero is the only one not present. As fully revealed in the second game, he died in his collapsed tower, so he wouldn't be around for a rematch.
Game Breaker: In the first game, Golden Sunny/Silver C-Cell, which can only be obtained in a Fetch Quest, and the hidden Robopon Scar, which is found only in the game's toughest dungeon.
Golden Sunny knows the Alpha moves, which are extremely strong. Dr. Zero's Robopon know them, too.
In Pokémon Ruby and Sapphire, a new feature was the ability to dive underwater. You used this to track down the villains. In the first Robopon, you can dive underwater as well—there's no bad guys there, but there's a health spa!
Most Annoying Sound: In the second game, a particular "song" is nothing but distorted synthesizers and a constant drumline, seemingly calculated to be as annoying as possible. While only played in a few, short scenes normally, it becomes a huge problem if the RNG hates you. Why? The mushroom spirit that changes the battle music changes it to this, for 33 battles.
Most Wonderful Sound: The VWEEP noise when you collect a hidden item in the first game, and the sparkling noises when you examine a teddy bear.
Surprisingly Improved Sequel: Robopon 2 did a much better job of breaking away from the Pokemon mold by making battles four on four, and introducing the love/hate oil system, in which Robopon of a specific oil type (either A, B, AB, O, or ?) "liked" Robopon of certain oil types and "hated" Robopon of other types. Using this system in tandem with any software on hand made it possible to teach almost any Robopon skills that could target every enemy on the field, or every one of your allies, and is an integral part of the Meta Game.
That One Boss: Mr. Wild and Prince Tail in the first game, Circe and Insector in the second.
Dr. Zero, both games.
In the first game, Kamat's army. Without being familiar with or good at the minigames that were optional up until this point, some players may have a hard time getting past the challenges, particularly the Jumper minigame, since it revolves solely around the Speed stat.
Optional boss Hunter. He has Golden Sunny.
Dr. Zero. Senior. Even if you thrashed Zero, Jr., Zeke, and the Bonus Bosses, he can kill you quite easily.
That One Level: In the first game, Grease Mountain is the proverbial brick wall. Things only get harder from there.
In the second game, the Galileo Windmills, Dreamless Island, the burning Waffle Tower, the post-crash Box Tower, the Neon City sewers, and the Pond Garden tend to frustrate players very thoroughly, due to either a) the enemies, b) the layout, c) bosses, or d) all three at once.
That One Sidequest: Averted in the first game's Elite 8 quest for Golden Sunny/Silver C-Cell. You'd think getting one of the game's Olympus Mons would be difficult, but all it is is a trading game. The Robopon you start the trade with, Gidget, is easy to find and catch, too.
Played straight with the Brownie sidequest. You go to a specific forest at around 5:00 p.m., and you have to arrange rocks to make several numbers. You're told which ones to make, but only a few rocks can be moved at all. And you do this while being attacked by wild Robopon.
Unintentionally Sympathetic: Dr. Zero in the second game, as he wonders how he lost, and when you realize his goal of world domination stemmed from his childhood dreams, which may or may not have come from seeing his future self fighting Cody. This may have been done on purpose to make Dr. Zero, Sr. even more evil.
The Woobie: Prince Tail, in the first game. First Dr. Zero kidnaps Princess Darcy. Then Tail loses to Dr. Zero, losing his Legend1 title to the evil scientist. After training at Cherry Hill to win her back, Tail loses to Cody, losing the title of Legend2 to him. Then Zero traps Princess Darcy in a mirror because Tail lost to him and you. Cue Heroic BSOD. To top it off, even after you beat the game, you don't have to save Darcy, and if you're playing on a ROM saving her becomes impossible.
Tail: I lost to Cody and lost everything that matters to me...