There are 27 secret characters in Lego DC Super-Villains that you can unlock by entering the secret code in the Joker's Funny House. To unlock the house, you need to complete the first mission. Inside the house, on the left side, there will be the face of the Giant Laughing Joker - interact with him and enter the code.
Note: unless you enter the cheat code after the first mission. you will need to complete the second task. This will open up a fast travel map, so head to the Joker's house by opening the Toothed Door.
Adam Strange - QF7NT
Atrocytus - 9HRJJ
Black canary - ND6AL
Blue Beetle - 8VV5Q
Clock king - Z6AU7
Detective Shimp - BQWSG
Dex Starr - KFLQM
Doctor Fate - KPTCC
Doctor Light - 6NWX5
Dr. Poison - GWWBS
Fire storm - HFMHM
General Zod - D8577
Green torch - KNJ2P
Lady Hawk - LNSB9
Jessica Cruz - S5DB6
Lady shiva - XQP2L
Martian hunter - Y7MFR
Mr Mallah - GM9MX
Mister Mixispitlik - NURPU
Plastic man - E6HUY
Rose Wilson - UVWHS
Red robin - JNLPY
Red tornado - CKDRF
Star sapphire - QD2GY
Terra - F79GU
Trickster - YRZMS
Toy maker - VB5AS