Here is the full list of all 123 Cassette Beasts achievements.
Open a door long since thought lost.
Get to 5 hearts with Viola
Get to 5 hearts with Eugene
Complete Frankie's training
Get to 5 hearts with Felix
Defeat 50 Rogue Fusions
Complete 20 noticeboard quests
Defeat an Unstable Fusion
Obtain 100 different species
Get to 5 hearts with Meredith
Complete Vin's training
Observe 120 different species
Obtain Candevil
Complete 5 noticeboard quests
Win a battle before it starts
Defy all odds in a place between worlds.
Get the Fusion Radar
Meet the Travelling Merchant
Defeat Ianthe and become a Ranger Captain
Defeat Lamento Mori
Defeat a ghost
Solve the riddle of Morgante's song.
Reopen every station
Defeat Babelith
Complete Morgante's song
Defeat Alice
Enter a romantic relationship
Defeat Captain Codey
Defeat 20 Rogue Fusions
Help Sunny start a new life
Defeat Captain Gladiola
Complete Eugene's quest
Clear 5 Landkeeper offices
Obtain the Averevoir Flight ability
Clear 4 Landkeeper offices
Visit every part of New Wirral
Defeat Captain Cybil
Complete Barkley's quest
Obtain an Ice-type bootleg
Unlock Barkley
Complete Felix's quest
Meet Kuneko at 4 altars
Meet Kuneko at 3 altars
Meet Kuneko at 2 altars
Meet Kuneko at 1 altar
Meet Kuneko
Defeat Captain Skip
Obtain the Pumpkin Vine Ball ability
Help Hoylake with his research
Clear 3 Landkeeper offices
Defeat Captain Buffy
Defeat Captain Lodestein
Clear 2 Landkeeper offices
Complete Viola's quest
Bring Viola to the shipwreck
Defeat Captain Heather
Obtain 50 different species
Unlock Viola
Defeat 10 Rogue Fusions
Get to 5 hearts with Kayleigh
Find a helping hand in a place far away.
Find yourself somewhere far away.
Defeat Heckahedron
Obtain the Diveal Swim ability
Complete Meredith's quest
Bring Meredith to the mall
Unlock Meredith
Clear 1 Landkeeper office
Unlock Eugene
Unlock Felix
Obtain an Astral-type bootleg
Defeat a Swarm
Defeat Captain Clee-O
Defeat 5 Rogue Fusions
Obtain the Electromagnetism ability
Defeat Captain Penny Dreadful
Try to sneak past a battler by jumping over their head
Defeat Captain Judas
Obtain 20 different species
Complete Kayleigh's quest
Choose a design for Harbourtown's sign
Defeat Captain Zedd
Obtain the Bulletino Dash ability
Investigate a rumour
Defeat Poppetox
Start Kayleigh's quest
Meet the one who dutily attends the stations.
Defeat Captain Wallace
Obtain a rumour
Sign up to the Rangers
Obtain 10 different species
Defeat a Rogue Fusion
Remaster a tape
Complete the tutorial
Obtain the Mothwing Glide ability
Unlock Kayleigh
Obtain Bansheep
Obtain a rare sticker
Defeat an Orb Fusion
Obtain 120 different species
Obtain additional data on every species
Obtain a Beast-type bootleg
Obtain a Fire-type bootleg
Obtain a Plastic-type bootleg
Obtain a Plant-type bootleg
Obtain a Water-type bootleg
Obtain a Poison-type bootleg
Obtain a Lightning-type bootleg
Obtain an Air-type bootleg
Obtain an Earth-type bootleg
Obtain a Metal-type bootleg
Obtain a Glass-type bootleg
Obtain a Glitter-type bootleg
Obtain a monster form thought impossible to find.
Get to 5 hearts with Barkley
Defeat an ancient force brought back to full strength.
Overcome the essence of fusion.
Come face-to-face with a faceless being.
Complete 50 noticeboard quests
Complete 100 noticeboard quests
Win a battle while ghostly
Win a battle while Gambit is active
Prove your worth to a certain warrior.