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.
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.
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.
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.