In order to obtain the gaiden chapters in the game, also called sidequests, you must fulfill certain requirements.

Chapter 5x: Use a door key to unlock the door to the empty room in chapter 5. Once it is unlocked, have any character step inside the room to obtain a Talisman. Upon finishing the chapter, the player will be met with the option to go to chapter 5x.

Chapter 7x: Beat chapter 7 in 15 turns or less. At the end of the chapter, the player will have the option to go to chapter 7x.

Chapter 17x: Beat chapter 17 in 32 turns or less. At the end of the chapter, the player will have the option to go to chapter 17x.

Chapter 19x: Talk to Rex with either Anakin or Kevin, thus recruiting him. At the end of the chapter, the player will have the option to go to chapter 19x.

Chapter 23x: Automatically upon completing chapter 23.

Chapter 23xx: Collect all four Emblem Weapons throughout the game. The Emblem Lance is found in chapter 2 by visiting a house to the south with Gordon. The Emblem Sword is found in chapter 8 by unlocking a chest in a room to the north. The Emblem Axe is received by defeating a Warrior in a western corner of chapter 23. The Emblem Bow is received by having Haas talk to Storm in chapter 23x. At the end of chapter 23x, if all four weapons were collected, one will proceed to chapter 23xx.

Chapter 25x: Beat chapter 25 in 35 turns or less. At the end of the chapter, the player will automatically go to chapter 25x, without being asked if they want to first.