At the arena select screen in VS. mode, hold the Z button on Basic Arena or No Man's Land to play with alternate graphics for those stages.
Athena
After completing the Bronze Handicap Match and getting all the silver trophies, not including the bronze handicap match, you will be able to compete in the Silver Battle Royal. You will not get any new parts for any of the matches except for the last one. Once you finish the last battle, you will get the Athena body part.
Cans
To get "Can Robo" Can Bomb, Can Pods, Can Boots, and Can Gun, win the Tournament in your house. You can now enter Lucy's room. In there you will see lots of people talking about what they did for the game (similar to "talking" credits). Go through the second door and you will see more people. In the second room there is a robot (not a Custom Robo). Talk to him after he is done talking. The game will save. Go to the nearest parts generator and get ready to laugh at what you just unlocked.
Chickenheart
In the Z Boss battle, look on the north side of the west pillar.
Completion Bonuses
To unlock Arcade Mode, First Person View, Parts Index, and The Grand Battle in Single, and Versus Mode CPUs, you must complete 'A New Journey' mode in single player.
Crystal Strike
Defeat the fourth CPU in in the Test Center, then keep talking to it until you get the Crystal Strike gun.
Dark Star versus mode stage
Play versus mode 50 times with any one Robo. For example, play vs. mode 50 times with Ray01.
Defeating large enemies
To defeat enemies such as the Metal Bear, Tank Head, etc., use the Peregrine with Formula Legs and either the Shotgun or the Knuckle. You must get close to your enemies to use these weapons, but the Peregrine is incredibly fast, and the Knuckle and The Shotgun are two of the game's most powerful guns.
First Person Mode
Beat a new journey in Story Mode.
Fresh Stage
Go to VS mode highlight the basic arena or No Man’s Land and press Z.
Grand Battle
Beat a new journey in Story Mode.
Holosseum data
Go to the bottom-left hand corner of the Holosseum. You should find data on the Holosseum.
Juggle your opponent
Every Robo has a dash attack that they can perform with . Some Robos have a straight-forward dash. If you are using one of those Robos, keep pressing X and you will juggle your opponent and do a good amount of damage. This is very useful if you have low health.
Link Bonuses
Use a Game Boy Advance link cable to connect to Custom Robo GX to unlock the Dark Core stage and new Robo parts in the Japanese version of the games.
Parts Index
Beat a new journey in story mode.
RahuIII
After you Beat Grand Battle Mode, go back to the 'Outside world' Amusment park. Enter the Silver Battle Royal and do not skip the senario. Before the match starts walk straight down until you find a broken carriage. You can enter the carriage on the right side. Inside tap A like crazy till you pick up a data chip. And now you have big, bad Rahu3.
Rakansen
Win all the Amusement Park Tournaments with bronze trophies.
Ray Legend
Get a gold trophy in all cups in the Z headquarters then talk to Oboro.
Ray Warrior
Get a gold trophy in all the cups in the Amusement Park, then talk to the policeman at the front gate.
Single use tournaments
To get gold in a single use tournament, do not use Illegal parts as they cut your points in half. Use the following combination, or something of the sort. In the first two to three battles use Metal Ape, Metal Bear, Metal Ox, with these guns: Afterburner (stand back and fire for an easy perfect), Blade (stand back, fire your opponent into a wall, and keep firing for an easy perfect), Halo (stand on ground and fire repeatedly for an perfect). Note: These guns do not too well in Ice Holloseums, In the case of Ice Holloseums, use a Javelin or Halberd type. In battles four through six, use the Peregrine and either Shotgun or Knuckle. Note: If there are more battles than six, use Peregrine and Knuckle, Swift and Shotgun, And Shrike and Flame. For the last battle, use your preferred Robo (for example, Ray 01) and your favorite weapon (probably the Dragon).
Super Mario Bros. reference
When the weird robot attacks Bogey's Café, defeat it then the blue-haired person. When Harry asks if you are going home, tell him that you will wander around. Then go to Hero's home talk to Lucy (the woman that sweeps with the broom), You will tell her about Z, and she will think that it is a band and that you were a security guard. Talk about the band. She used to hear the Groovy Goombas and their hit song "Fire Flower Sunshine".
Useable CPUs in VS battle
Beat a new journey in Story Mode.
Wave Laser
Go to the Lab Handicap battle, then press A against the books. You will get the Wave Laser gun illegal part.
X Laser
To find the X Laser (illegal part), in a Grand Battle mode, go to Bogey's Café. Walk behind the counter and select the shelf. You will now have an illegal part.