EDIT: I made this guide because I didn't know how to code a tool at the time. Now I do, and I made a tool here. Feel free to continue following this guide if you want, but know that the tool is also available to do it for you.
Well, first tutorial I've ever done (for anything, EVAR) here goes.
1) First, extract your Sonic Adventure Chao save from your 360. However you want to do this doesn't make a difference, either hook your HDD up directly to your computer or use a UBS flash drive.
2) Open the save data in a hex editor. Even though I hear many sub-par things about it, I prefer Modio for all my modding needs, and it works perfectly for editing this game since everything you'll need is included right in the program. (This guide assumes you are either using an all-in-one package like Modio, or are using separate programs to accommodate.)
3) No checksum or encryption/compression to deal with here. Just simple values to edit. Of course, the Chao system is pretty deep, so often times those more unique values can be a little harder to find if you don't already know where they are, so here's a picture to show you what you're looking for. Just scroll down until you see something resembling this data pattern. Be careful; my example is a pretty complex one, so yours more than likely won't look exactly like it and may even have plenty of 0s when compared to it, though it will be structurally identical.
[SPOILEROPTION="Open for graph data."]
This pattern shouldn't be too difficult to find. The file is kinda small, so it only holds the header info and then immediately jumps to Chao data like this. Each of of these little chunks is a single Chao, listed in order by which came into existance first (ie, when the egg was found/bought). So listing the circled values as letters, here's whats what:
A: Grade These go from 0 to 5, 5 being the highest. Grade is how high each stat boosts upon level up. The three values after are also grade levels, but I'm not certain what they are (I know they are luck, intelligence, and mood, but these are hidden values and thus difficult to test; I don't know which order they're in, so I left them blank).
B: Level Same order as before (hidden stats too, if you can figure them out). The highest stat possible in-game is 99, but inputting 255 (FF) will work without any issues.
C: Stat Value Again, same order. This is the important one out of the ones mentioned so far, as this is the one that actually dictates your Chao's abilities. Highest possible value in-game is 3266, but just like levels, maxing it out won't kill your game. Just be prepared for a Chao that is so skilled that he goes full circle and actually cannot finish certain races. *sad face* Also, if you edit this value and your Chao levels up then the stats will revert to what they should be. Avoid this by changing the level to or above 99 so that it can't level up anymore.
D: Chao Type This is what your Chao wlll actually look like. Don't want to go through all the trouble to reincarnate 2 times and give it every animal? Edit here.
E: Garden Which Chao Garden is this particular Chao in?Code:00 - No Chao (does not exist, rendering ALL other mods for that Chao null.) 01 - Egg (Chao exists, but is still an egg. Haven't tested whether this nulls other edits.) 02 - Child/Normal 03 - Invisible (unsure exactly what else changes) 04 - Invisible (again, not entirely sure) 05 - Neutral/Normal 06 - Hero/Normal 07 - Dark/Normal 08 - Neutral/Swim 09 - Hero/Swim 0A - Dark/Swim 0B - Neutral/Fly 0C - Hero/Fly 0D - Dark/Fly 0E - Neutral/Run 0F - Hero/Run 10 - Dark/Run 11 - Neutral/Power 12 - Hero/Power 13 - Dark/Power 14 - Neutral/Chaos 15 - Hero/Chaos 16 - Dark/Chaos 17 - Tails 18 - Knuckles 19 - Amy
F: Reincarnation Status Either it will or it won't. FF says it will reincarnate after it dies, anything else says no.Code:00 - None (I haven't test this. I assume it nulls all edits.) 04 - Station Square 05 - Egg Carrier 06 - Mystic Ruins (I don't know what 01-03 does, I assume the same as 00.)
G: Eye Type I haven't played with this. Toy around with it.
H: Mouth Type I haven't played with this one too much either. I know 00 is no mouth, but I'm afraid that's the extent of my current knowledge on this one.
I: Ball Type The little ball that floats above Chao's head.
J: Hat Type Holy Chao hats, Batman. There are A LOT of possibilities for this one. Get ready for this one.Code:00 - Normal 01 - Flame 02 - None
[SPOILEROPTION="I'm serious, this is A LOT of info."][/SPOILEROPTION]Code:00 - None 01 - Pumpkin 02 - Skull 03 - Apple 04 - Bucket 05 - Can 06 - Box 07 - Flower Pot 08 - Paper Bag 09 - Brass Pot 0A - Tree Stump 0B - Watermelon 0C - Red Beanie 0D - Blue Beanie 0E - Gray Beanie 0F - Pacifier (Glitchy) 10 - Normal Egg 11 - Yellow Egg 12 - White Egg 13 - Brown Egg 14 - Cyan Egg 15 - Pink Egg 16 - Blue Egg 17 - Dark Gray Egg 18 - Dark Green Egg 19 - Red Egg 1A - Light Green Egg 1B - Purple Egg 1C - Orange Egg 1D - Black Egg 1E - Yellow Egg with Spots 1F - White Egg with Spots 20 - Brown Egg with Spots 21 - Cyan Egg with Spots 22 - Pink Egg with Spots 23 - Blue Egg with Spots 24 - Dark Gray Egg with Spots 25 - Dark Green Egg with Spots 26 - Red Egg with Spots 27 - Light Green Egg with Spots 28 - Purple Egg with Spots 29 - Orange Egg with Spots 2A - Black Egg with Spots 2B - Shiny Normal Egg 2C - Shiny Yellow Egg 2D - Shiny White Egg 2E - Shiny Brown Egg 2F - Shiny Cyan Egg 30 - Shiny Pink Egg 31 - Shiny Blue Egg 32 - Shiny Dark Gray Egg 33 - Shiny Dark Green Egg 34 - Shiny Red Egg 35 - Shiny Light Green Egg 36 - Shiny Purple Egg 37 - Shiny Orange Egg 38 - Shiny Black Egg 39 - Shiny Yellow Egg with Spots 3A - Shiny White Egg with Spots 3B - Shiny Brown Egg with Spots 3C - Shiny Cyan Egg with Spots 3D - Shiny Pink Egg with Spots 3E - Shiny Blue Egg with Spots 3F - Shiny Dark Gray Egg with Spots 40 - Shiny Dark Green Egg with Spots 41 - Shiny Red Egg with Spots 42 - Shiny Light Green Egg with Spots 43 - Shiny Purple Egg with Spots 44 - Shiny Orange Egg with Spots 45 - Shiny Black Egg with Spots 46 - Gold Egg 47 - Silver Egg 48 - Ruby Egg 49 - Sapphire Egg 4A - Emerald Egg 4B - Amethyst Egg 4C - Aquamarine Egg 4D - Garnet Egg 4E - Onyx Egg 4F - Peridot Egg 50 - Topaz Egg 51 - Purple Swirl 52 - Deep Shiny Sky Blue 53 - Deep Shiny Light Sky Blue 54 - Dark Chrome (I hated typing all that... *sad face*)
K: Feet Toggle Yeah, you can cut off their feet too.
L: Medal Chao can win medals in the races. Use this to edit a specific medal you want Chao to wear.Code:00 - Feet 01 - No feet (Chao just floats around like a ghost.)
M: Chao Color Here's the fun one. Chao come in many different colors, but why bother keeping track of it all when you could just hack the one you want in?Code:00 - None 01 - Aquamarine 02 - Topaz 03 - Peridot 04 - Garnet 05 - Onyx 06 - Diamond 07 - Beginner Race 08 - Challenge Race 09 - Hero Race 0A - Dark Race 0B - Pearl 0C - Amethyst 0D - Emerald 0E - Ruby 0F - Sapphire
N: Highlights Toggle There is also a way to get multicolored Chao. This enable that option. Note that regular Chao have this automatically turned on while colored Chao have it turned off. Works strange with Shiny/Jewel Chao.Code:00 - Normal 01 - Light Yellow 02 - Light Gray 03 - Brown 04 - Cyan 05 - Pink 06 - Blue 07 - Dark Gray 08 - Dark Green 09 - Red 0A - Light Green 0B - Purple 0C - Orange 0D - Black (The game accepts high values and won't crash, but these are the only ones I tested extensively. Try out more and see where it gets you.)
O: Shiny Toggle Just like Pokémon, Chao can be Shiny on top of whatever color they happen to be. Works normally on Colored Chao, strange effects on regular/Jewel Chao.Code:00 - On 01 - Off
P: Egg Color Only important if Chao is still in Egg form. Doesn't affect actual Chao color. Refer to Chao colors for values.Code:00 - Off 01 - On
Q: Chao Model Actually swaps out the in-game model used for Chao. Hacking is the only way to change this. Changing this to anything except "Chao" nulls all appearance mods.
Q: Relation to Characters This is Chao's relationship status to the player's characters in order: Sonic, Tails, Knuckles, Amy, Big, E-102γ. 00 is normal, FF is highest.[/SPOILEROPTION]Code:00 - Chao 01 - Egg Chao (An unhatched egg with eye holes and feet) 02 - Omochao
4) After the desired changes are made, resign/rehash save file and inject it back into your HDD/USB flash drive.
5) Play your game! It may take a little longer to load the first time. If it does, don't worry about that. Of course, it shouldn't take more than 30 seconds (give or take), so if the game actually freezes then go ahead and post your problem (and save file) here and I'll see what I can do to help. ^-^ Side note, remember that editing your Chao's stats extremely high WILL cause it to be unable to finish certain races. While editing Chao's stats to FFFF won't crash your game, I'd advise against it, at least until you've already won all the races with that Chao.
My tutorial only goes over about half of what is editable. I was looking into the rest for a while, but to be honest I lost interest and stopped researching. If you want to pick up where I left off, please post any findings on this thread. I'll see about adding any findings (and, of course, giving credit) so if you want to pick it up, please, be my guest. ^-^