SCHG How-to:Add Extra Characters To Sonic 2

(Guide By Manic Remix)

(Revised by ThomasSpeedrunner)

I was very surprised that this guide didn't exist anywhere else. It seems like nowadays this is the "what everybody has in their hacks" type of thing. This guide will show you how to add extra characters to Sonic The Hedgehog 2 (16-bit). We will use the SVN/Hg disassembly. In this guide, I will refer to Knuckles and Amy.

First, open up s2.asm. Search for "Obj_Index:".

You will see this or something like it (ignore the fire shield reference as I copy-pasted this from Sonic 2: Titanium Upgrade; Obj09 will be Special Stage Sonic in your hack unless, like me, you've gone and deleted the special stage code and objects):

; ===========================================================================
; ---------------------------------------------------------------------------
; OBJECT POINTER ARRAY ; object pointers ; sprite pointers ; object list ; sprite list
;
; This array contains the pointers to all the objects used in the game.
; ---------------------------------------------------------------------------
Obj_Index:
ObjPtr_Sonic:			dc.l Obj01	; Sonic
ObjPtr_Tails:			dc.l Obj02	; Tails
ObjPtr_PlaneSwitcher:		dc.l Obj03	; Collision plane/layer switcher
ObjPtr_WaterSurface:		dc.l Obj04	; Surface of the water
ObjPtr_TailsTails:		dc.l Obj05	; Tails' tails
ObjPtr_Spiral:			dc.l Obj06	; Rotating cylinder in MTZ, twisting spiral pathway in EHZ
ObjPtr_Oil:			dc.l Obj07	; Oil in OOZ
ObjPtr_SpindashDust:
ObjPtr_Splash:			dc.l Obj08	; Water splash in Aquatic Ruin Zone, Spindash dust
ObjPtr_FireShield:		dc.l Obj09	; Fire Shield
ObjPtr_SmallBubbles:		dc.l Obj0A	; Small bubbles from Sonic's face while underwater... 

And it will go on in this format. Now, search for "dc.l ObjNull". Replace this:

				dc.l ObjNull	; Obj4C
				dc.l ObjNull	; Obj4D

with this:

ObjPtr_Knuckles:		dc.l Obj4C	; Knuckles
ObjPtr_Amy:			dc.l Obj4D	; Amy

You've now created a Knuckles object and an Amy object. These are the object IDs that will be used for your two new characters.

Next, search for "InitPlayers:". You will see this or something like it:

 
        InitPlayers_TailsAlone:
	move.b	#ObjID_Tails,(MainCharacter).w; load Obj02 Tails object at $FFFFB000
	move.b	#ObjID_SpindashDust,(Tails_Dust).w; load Obj08 Tails' spindash dust/splash object at $FFFFD100
	addi.w	#4,(MainCharacter+y_pos).w
	rts 

Now, below:

InitPlayers_TailsAlone

add

	subq.w	#1,d0
	bne.s	InitPlayers_KnuxAlone			; branch if this is a Knuckles alone game

Now, below the "rts", add

InitPlayers_KnuxAlone:
	subq.w	#1,d0
	bne.s	InitPlayers_AmyAlone			; branch if this is an Amy alone game
	move.b	#ObjID_Knuckles,(MainCharacter+id).w
	move.b	#ObjID_SpindashDust,(Sonic_Dust+id).w
	rts

InitPlayers_AmyAlone:
	move.b	#ObjID_Amy,(MainCharacter+id).w
	move.b	#ObjID_SpindashDust,(Sonic_Dust+id).w
	rts

Now, go to s2.constants.asm. Find this line:

ObjID_Buzzer =			id(ObjPtr_Buzzer)		; 4B

And insert this under it:

ObjID_Knuckles =		id(ObjPtr_Knuckles)		; 4C
ObjID_Amy =			id(ObjPtr_Amy)			; 4D

Next, go to "OptionScreen_Choices:", and find this line:

	dc.l (3-1)<<24|(Player_option&$FFFFFF)

Change the 3 to a 5. This will allow for Knuckles and Amy player options. Now go here:

off_92D2:
	dc.l TextOptScr_SonicAndMiles
	dc.l TextOptScr_SonicAlone
	dc.l TextOptScr_MilesAlone

And add this under it:

	dc.l TextOptScr_KnuxAlone
	dc.l TextOptScr_AmyAlone

Now, head here:

off_92DE:
	dc.l TextOptScr_SonicAndTails
	dc.l TextOptScr_SonicAlone
	dc.l TextOptScr_TailsAlone

Add the same two lines as before under:

	dc.l TextOptScr_TailsAlone

Now find this:

TextOptScr_TailsAlone:		menutxt	"TAILS ALONE    "	; byte_981C:

And add this below it:

TextOptScr_KnuxAlone:		menutxt	"KNUCKLES ALONE "
TextOptScr_AmyAlone:		menutxt	"AMY ALONE      "

And there you have it! You have two new characters. Now you will need to make new sprites, palette, and code for them to work. A good start would be to copy and paste Sonic's code into a new ASM file and change the labels so you don't get "symbol double defined" errors. Happy hacking!

|Add Extra Characters To Sonic 2]]