Save files and settings are in your documents (where you looked). Game files (the models, textures, scripts, config data, etc) depend on where you installed the game, refer to unknown's post if you got the game on steam or through the CD version assuming you used the default installation directories.
Ide.img is the file you'll want to modify for making any characters spawn differently. Ide.img is actually an archive composed of multiple files, but it is easier to simply modify ide.img itself instead of extracting what you need since it is quicker and less complicated for new modders. Ide.img is located in /Bully Root/Objects.
The file you want in ide.img is default.idb. Default.idb is some sort of compiled or encrypted version of default.ide (I'm not sure how exactly the IDE files were converted to IDB, nor the reason for it. This however was something done by Rockstar, not fellow modders). The default.ide file isn't really needed by the game, so it is a wonder Rockstar left it in the files, but it is nice they did (it is not in ide.img, but it is in the same directory). Default.idb is what the game actually uses (and is in ide.img), and this file is used by the game to setup all the ped (character) settings. This includes multiple things, but the one you'll be looking for is the "unique model status" setting. Now in default.idb, it may seem hard to tell where that is as there is not any labels, but it is indeed findable.
First, open your ide.img file using
HxD. The contents of default.idb should be the first thing you see. The rest of the files in the ide.img archive are here too but they are after default.idb, you do not need to worry about them now though.
Second, identify the ped you want. Each ped is listed here and contains several things. I'll put next to each thing the value the player (Jimmy) has so you can see in your default.idb what I'm talking about and where it is located.
Model: player
Winter texture: player
Size: Medium
Faction: PLAYER1
Stat: STAT_PLAYER
Animation group 1: Grap
Animation group 2: null
Animation group 3: null
Animation group 4: null
Unique model status: ÿÿÿÿ
Action tree root: /Global/Player
Action tree file: Act/Player.act
AI root: /Global/PlayerAI
AI file: Act/PlayerAI.act
Name: N_Jimmy
To find the ped you want, you'll have to identify their model or name. HxD has a find option, so you can find what you need easily by pressing ctrl+F and entering what you want to find. For example pressing ctrl+F and typing in "Johnny" will help you find "Johnny".
Now that you found the ped you want, find its unique model status. Action trees always start with "/Global" and all these settings are in order, so you should be able to find the unique model status right before that first "/Global" (the second "/Global" will be for AI, make sure not to confuse the two). The unique model status is a number that tells the game how many occurrences of a specific model there can be. If the number is -1, then that means there can be none. If it is 1, then that means there can only be 1 of that model. If it is 2, then there can be multiple. Now you're probably thinking "but I don't see a -1, 1, or 2", and that is correct, you do not. This is because the number wasn't stored in plain text (it was in default.ide but not default.idb), so it looks a little funny. Numbers are stored in memory a little differently than text and since I don't want to over complicate things for you, I'll just tell you what you need to type for each value. The unique model status should always be 4 bytes long though since that is what the game expects the size of the number to be. The text on the right of the screen is all the bytes in the value represented by characters (since a single text character typically is only one byte), and on the left is the value of each byte in
hexadecimal. To change one of the unique model statuses, select the start of the 4 byte sequence on the left so you can edit the values in hex, and type one of the following sets of numbers (the numbers should be replaced not added, this is the default behavior HxD exhibits when you start typing).
FF FF FF FF = -1 (never spawn)
01 00 00 00 = 1 (spawn one copy)
02 00 00 00 = 2 (spawn multiple)
There may be multiple models for one character, make sure you modify the unique model status for all of them. For example Fatty has his normal model (NDH1_Fatty), his wrestling model (ND_FattyWrestle), and a chocolate version (NDH1_FattyChocolate).
Now that you are done, save your changes (ctrl+S), close the file, and start the game. You should hopefully be able to see that your changes were successful.