Lenny (Toy Story): Unlock Buzz, Woody, Bo Peep, Slinky, and Rex, AKA the Toy Story 1 set of characters. Once you unlock them, then Lenny will unlock automatically.
Jessie (Toy Story): First, unlock Bullseye the horse. Then, you have to collect 50 cherries. It can be over the course of many rounds or over one round. Then, finish the level and Jessie will unlock automatically.
Fred Super Suit (Big Hero 6): Unlock the Super Suit set of characters. This includes Hiro Super Suit, Honey Lemon Super Suit, Wasabi Super Suit and Gogo Super Suit.
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Bobby (Inside Out): Unlock Bobby, Dave, Paula and Bing Bong, also known as the Brain Maintenance set. Once you unlock this whole set, Bobby will automatically unlock.
Fritz (Inside Out): Unlock the Feeling Emotional set, which includes Joy, Anger, Sadness, Disgust and Fear (in no particular order). Unlock all of them and you will get Fritz as well.
Gus (Haunted Mansion): Unlock the Spooky Spirits set to unlock Gus. This includes Prof. Phineas Plump, Madame Leota, and the Executioner. Once you do, Gus will automatically unlock.