Custom Robo Arena is a RPG/Action hybrid game based around the customization of the titular Custom Robos for use in combat.
The single player mode of Custom Robo Arena is very similar to a Pokemon game. Players progress through a stereotypical RPG storyline while collecting and purchasing parts to increase their customization options . Arena heavily emphasizes upgrading your own Robot's five types of parts (Body, Gun, Bomb, Pod, and Feet) and with the large number of parts available, there are plenty of options. There is a rank system that levels the player up for optional battles, but it has no actual effects on the player's Robo and only becomes plot relevant twice in the game (both times require the player to have a certain ranking before proceeding). There is a routine built up by the game's plot, every day the player takes the same actions, but there is a side-quest (hunting criminals) that takes place at a time the player has been taught that nothing happens, which can lead many players to miss that aspect of the game. There is also a new cleaning aspect of the game, requiring the player to clean her Robo parts after battle, or deal with reduced stats.
Combat in Custom Robo Arena is similar to Virtual-On. It takes place in an arena (called a Holosseum in universe) with various obstacles. Robos have different movement options based on their class, but all of them can run, jump, and preform a limited number of “air-dashes” (double-jumps, except in the two classes that fly). Each Robo has four attacks, their primary fire, determined by their gun, generally rapid but weak, secondary fire, determined by the Robo's bomb, generally slower but stronger and spreads, their pod, which is a trap which either lies in wait for or chases the opponent, and their dash, a physical attack determined by their model. Arena also adds a new ability, the Soulboost. When the player activates it (by touching the screen in combat) his Robo turns gold for a short time, increasing all of its abilities for a short time. However, after that time has elapsed, the Robo will become very dirty for another short period, making it vulnerable. As such Soulboost is fairly risky. In single-player, it is plot-relevant, so only boss-level characters will have access to it. All players can use Soulboost in multiplayer. An important mechanic in Custom Robo is Downing the enemy Robo. When a Robo takes a certain amount of damage (depending on the model) it will be Downed. This stuns the Robo, making it vulnerable. Equally important is the fact that a Downed Robo's primary fire vanishes, making Downing viable defensively as well. After a Robo is downed, they have a short period of invulnerability to avoid being trapped in a loop.
One interesting feature in Arena is that it adjusts its difficulty to the player's skill. If a player fails multiple times on the same battle in single player, the game will reduce the opponent's HP--first by 25%, then 50% and finally 75%.
Custom Robo Arena has online battles, which are similar to single-player but against a real person. The player can use all of the parts and arenas she has purchased in single-player in the online mode. The player can also purchase “dioramas” and pose his Robo. The final pose is shown to the player's opponent before the match begins.
This game was originally going to be a GBA game similar to Custom Robo GX, but it was ported to the DS. The RPG sections were all designed to run on a GBA.