Sonic Adventure 2 is a 3D platform game divided into two campaigns: Hero and Dark. In the Hero campaign, players control Sonic, Tails, and Knuckles, who fight to save the world; in the Dark campaign, players control Shadow the Hedgehog, Doctor Eggman and Rouge the Bat, fighting to conquer it. Each campaign cycles through levels of its three characters, telling different sides of the story. Levels have a variety of themes (such as cities, jungles, desert pyramids and outer space),[3] with some followed by boss fights. The two campaigns' stories occur in parallel; completing both campaigns unlocks a Last Story with all six characters, culminating in a final boss fight.[4]
Sonic and Shadow play fast-paced levels, emphasizing platforming.[5] Their homing attack can lock onto robots created by Eggman and G.U.N., and they can grind on rails. Tails' and Eggman's levels are slower and oriented towards multi-directional shooting; they are confined to mechs in which they can jump short heights, hover and shoot enemies. Knuckles' and Rouge's levels are open and feature action-adventure gameplay with treasure hunting; in each level, they must find three shards of the Master Emerald. Their search is guided by radar and puzzle-based clues from harmless robots.[4] Knuckles and Rouge can glide, defeat enemies with punches and kicks,[5] and scale walls, digging into them to find power-ups.[6]
Adventure 2 has the health system found in many other Sonic games. The player collects rings scattered throughout the levels; being hit by an enemy while holding rings causes the player to drop them all, while being hit without rings causes them to lose a life. Tails and Eggman have health bars, which are slowly refilled by collecting rings. Dying with no lives results in a game over screen.[7] The characters can obtain permanent upgrades that grant them new abilities; for example, one allows Sonic and Shadow to dash along a sequential trail of rings to reach distant platforms, while another lets Knuckles and Rouge attack powerfully enough to break certain containers.[8]
Separate from the main campaigns, the player can raise Chao as virtual pets.[4] They have five attributes (Swim, Fly, Run, Power and Stamina) and a moral continuum from Hero to Dark. From the moment they hatch their stats can be increased with Chaos Drives or small animals, found in the main stages, which empower them to compete in karate[9] and racing minigames.[5] Their alignment gradually changes, based on their affection for the characters; for example, a Chao which likes Tails will gradually become more heroic. Playing with Chao increases affection, and when a Chao becomes fully Hero or Dark it assumes that form permanently.[10] Although Chao eventually die, if they receive enough affection during their lives they reincarnate.[11]
Adventure 2 has 180 emblems, earned for a variety of tasks.[12] Each level has five missions; only the first is required to continue the campaign, and other missions include completing a harder version of a level and collecting 100 rings. The player earns emblems by completing missions and other tasks, many related to Chao raising. Collecting all the emblems unlocks a 3D version of the Green Hill Zone stage from the original Sonic the Hedgehog.[13]
The game has several two-player modes. Players may race on foot through new (or altered) levels, have shoot-'em-up battles in mechs,[5] hunt for Master Emerald shards[14] or race in go-karts.[5] A few characters are playable in these modes, but not in the main game; Tikal and Chaos from the original Sonic Adventure are playable in the treasure-hunting game,[15] as are Amy Rose and Metal Sonic in the foot-racing levels[16] and mechs piloted by Chao and Big the Cat in the shooting levels.[17]
