Celeste

Story. Finish Chapter 1: Forsaken City, the opening chapter. Unlocks at the chapter end.

Grab any one of the hidden cassette tapes (there's one per main chapter). A chapter's cassette unlocks its B-Side; this pops on your first.

Story. Finish Chapter 2: Old Site (the dream-block and Badeline chapter).

Story. Finish Chapter 3: Celestial Resort (Mr. Oshiro's hotel).

Story. Finish Chapter 4: Golden Ridge (the wind chapter).

Collect 30 strawberries (cumulative across all chapters). Golden strawberries aren't required for any achievement.

Story. Finish Chapter 5: Mirror Temple.

Story. Finish Chapter 6: Reflection (the confrontation with Badeline).

Story. Finish Chapter 7: The Summit - reaching the mountain top completes the base game's main climb.

Each chapter hides a single Crystal Heart, and grabbing Chapter 2 (Old Site)'s unlocks its brutal B-Side remix. It sits in an optional room off the main path that takes some tricky dashing to reach. A collectibles map or video is the quickest way to the exact spot.

Collect the hidden Crystal Heart in Chapter 1 (behind a puzzle, off the main path). Replayable from chapter select.

Finish Chapter 9: Farewell, the free post-game chapter (reached from the overworld after the Core / Epilogue). The hardest chapter in the game - a long precision gauntlet.

Strawberries are purely optional bragging-rights collectibles - they unlock nothing mechanical - and there are 175+ across the game, so 80 is well under half. Grab them as you explore. The key rule: a berry only counts once you land safely / finish the room after touching it, so if you die before touching solid ground it doesn't bank. Winged berries fly off if you go down, so nab those on the way up.

The Crystal Heart in Chapter 4 (Golden Ridge), unlocking its B-Side. Hidden off the critical path in a room that takes some tricky dashes to enter. Use a map for the pixel-precise spot.

Chapter 3 (Celestial Resort)'s Crystal Heart, which unlocks its B-Side. Like all the hearts it's tucked in a hidden, optional room that needs a bit of precise platforming. Follow a collectibles map for the exact location.

Complete the Chapter 2 B-Side.

Complete the Chapter 1 B-Side (a remixed, much harder version, unlocked by collecting Chapter 1's cassette).

Beat the hidden PICO-8 "Celeste Classic" minigame. Access it via the computer in Chapter 3 at the "Huge Mess" checkpoint, then select PICO-8 from the main menu and climb to 3000m. Assist Mode doesn't work here, so it must be done legitimately.

Collect the Crystal Heart in Chapter 8 (the Core). Chapter 8 has no separate completion achievement - this heart is its marker.

Collect the Moon Berry, the secret strawberry hidden in Chapter 9: Farewell. On the final screen, after breaking the Vault keep going LEFT to a cluster of floating rock platforms and climb to the berry above the middle one.

The Crystal Heart in Chapter 6 (Reflection), unlocking its B-Side. Hidden in an optional room that needs some confident dashing to reach. Follow a map or video for the exact route in.

Complete the Chapter 3 B-Side.

Chapter 5 (Mirror Temple)'s Crystal Heart, which opens its B-Side. It's in a hidden optional room; the Mirror Temple's layout makes it especially easy to walk past, so a collectibles map is worth it here.

Complete the Chapter 4 B-Side.

Chapter 7 (The Summit)'s Crystal Heart, which unlocks its B-Side. Tucked away off the main climb in a tricky optional room. A collectibles map is the fastest way to pin the spot.

Complete the Chapter 5 B-Side.

Complete the Chapter 6 B-Side.

Complete the Chapter 7 (Summit) B-Side.

Complete the Chapter 8 (Core) B-Side.

Also in the PICO-8 minigame. Touch 6 different strawberries so they trail behind you WITHOUT landing on safe ground to bank them, then land to collect all 6 at once - you'll hear the 1UP jingle. Stay airborne or on walls / moving blocks between berries.

Collect 175 strawberries - effectively every standard strawberry in the game.

Beat all eight C-Sides - short, expert-tier remix challenges, each unlocked after collecting a chapter's Crystal Heart AND clearing its B-Side. One of the hardest achievements in the game.