Sonic Adventure (DC/GC, etc., 1998)

Six different playable characters (unlocked as you find them throughout the game, not everyone can play each stage but they tend to have unique challenges to beat in each one that they can play)
Can replay beaten stages
Some hidden permanent upgrades (light speed shoes (lets you reach some new areas and the casino level - the game guides you towards these though) and crystal ring which reduces the charge time plus ancient light (use light speed attack on enemies) for Sonic, jet anklet and rhythm badge for Tails, fighting gloves for Knuckles which sadly aren't that useful, warrior feather (spin hammer attack) for Amy, jet booster and gun upgrade for Gamma, swimming and others for Big)
​Auto-saves between stages
Unlimited continues
Interact with items (pick up/put down/throw, use as keys), you can pick up NPCs as well
No hub world map to get a better overview, Basic radar system for Knuckles' levels
One hidden stage plus some rooms and items
A diary of sorts recaps what you've done in previous game sessions
Chao garden (Tamagotchi-style mini-game, collect the Chao throughout the game)
Time limited levels as Gamma
Some sequence breaking with the unintended (?) super jump (power up a spin dash at the bottom of a hill, let go and jump immediately)?
Points of no return in stages
​Need to redo all the in-between stages' exploration with Tails
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Playthrough - Video Review (story spoilers) - Mini Review