#1 Adult Nala - Action Unlock!
Play as any character from the new Jungle Book movie. If you get struck by lightning twice you unlock Adult Nala after the the game is over.

 

#2 Mufasa - Action Unlock!
Play as Simba and spent at least 12 seconds near stampedes to unlock Mufasa. You have to spend as much time as possible right on the ground where the wildebeests (gnus) run. Hide behind a rock to not get run over.

 

#3 Hippo - "Humble Beginnings" Set
Characters: Simba, Nala, Rafiki, Zazu (Lion King) - This resembles the calm and peaceful beginning of the beginning of the Lion King movie.

 #4 Giraffe - "Big and Tall Characters" Set
Character Guesses: Jerry Jumbeaux Jr ., Willy the Giant (Zootopia), Executiner, Wreck it Ralph and more.
Unconfirmed. Let us know in the comments what worked for you!

 

#5 Scar - Action Unlock!
To unlock the Lion King secret character Scar, you have to get Mufasa first (by playing Simba and survive more than 12 seconds on the Gnu Stampede). Now you have to play Mufasa get back to the Gnu Stampede and get hit by a Gnu. This resembles the "Long live the king" scene, in which the villain Scar killed Mufasa. This will unlock Scare, who is the evil uncle of Simba and brother of Mufasa.

 

#6 Ed the Hyena - Daily Missions
This character can be unlocked by collecting 6 daily mission stamps.

 

#7 Blue Beetle - Action Unlock!
Play as Pumba (The Lion King) and collect 50 grubs. 
 Added in the Monsters INC update.

 

#8 The Rhino - Action Unlock!
Play as Bailey and collect 20 Fish Food. When you die, The Rhino will be unlocked.

 

NEW #9 Flower Mane - Action Unlock! 
Play as Simba and find the Flower Mane. Once you die, the character will be unlocked!