Secret Area before Giga Skullmageddon:
On the final stage of the game, after becoming a Ro-Bro, head left until reaching the left side of the screen. When you do, head down. You will climb down a hidden ladder with multiple 1-ups, and two unlockable chests.
Concept art gallery:
The Concept Art Gallery will unlock on the main menu after you complete the game, which contains 97 pages of unused concepts, drawings, and early game screenshots.
On Stage 7, "Some kind of lab", you will start near a punching bag. Attack the bag EXACTLY 87 times (1987 being the year Double Dragon came out) and you will hear a chime indicating you unlocked something. Quickly stop and the game will teleport you to a different area of the stage full of enemies, traps, and platforms. As an added easter egg, all characters, including the Lee brothers, have Yoshihisa Kishimoto's face instead of their normal face. He is the one that developed and made the original Double Dragon. Once you complete the area, the secret achievement "A Foot Face?" (5) will unlock.
Dragon and Double Dragon difficulties:
Complete the game in "Normal" difficulty to unlock Dragon difficulty. Then, complete the game under the Dragon difficulty to unlock Double Dragon difficulty.
Robot Form costume:
At the level selection screen, hold L1 + L2 + L3 + R1 + R2 + R3 + SELECT + START. A sound will confirm correct code entry. Note: This gives you the robot form costume without the extreme power up.
Setting Fuzz Face free:
Fuzz Face is a weird, flying friend with a very high-pitched voice. Set him free by ducking seven times between the two tombstones right beside the ladder in stage 7. He will then follow you around dropping Dragon Kick and batteries every minute or so, making the game much easier. Just be warned that his voice is quite annoying.
Secret Roper level:
During stage 7 where you start off with the heavy bag in the glass cell, punch the bag 87 times to hear a chime and wait a couple seconds. Your head will be replaced with Roper's, and all enemies in the stage also has their head swapped as well. Warning: this stage is very tough.
A Foot Face? - Become the hidden idol!
FuzzFace!! - Befriend everyone's most favoritest helpful character!
Gleam On! - Dodge and get the Gleam 50 times
Metal Head - Upgrade a song at the Tapesmith
Mr. Perfect - Complete any level without taking damage
Pick a Winner! - Nail all enemies with the hair pick
This Channel Sucks - Break all the Skullmageddon monitors in the Rocket Dojo
Victory! - Defeat Skullmageddon!
Weapon Master - Use every weapon once
Double Dragon - Finish every stage cooperatively on Double Dragon difficulty with friendly fire enabled!
Misters Perfect - Complete any level in co-op without taking any damage. . . with friendly fire enabled!
Single Dragon - Finish every stage solo on Double Dragon difficulty!
Max Fidelity - Max out every song to level 50!!