OVERVIEW:

Skip these three paragraphs if you don't care about the technical details.
Neo Mario uses a completely unique physics system to other Rivals, and as such he has some quirks, one of which is that he is very rarely in the actual hitstun state - throughout 90% of the match, Neo Mario is technically performing a neutral special that never ends. The main consequence this will have is that he doesn't tend to cooperate with command grabs.
Another thing to keep in mind is that to accomodate for the massive library of sprites he needs for his power-up system, Mario has been given unique rendering code. As a result, workshop characters that try to use their opponents' sprites won't be able to check Mario's. He is affected by shaders, but most of his sprites' outlines aren't black so AFAIK things that change his outline (i.e. Zetterburn's fire, Papyrus's blue attack) won't affect his outlines. Because he uses his own physics system, Mario's base Rivals variables such as dash speed, jump height, etc do not accurately reflect his stats nor are they actually used during normal gameplay, so characters that check or modify these variables will fail, however this shouldn't cause crashes.
Other than those quirks, he is designed to be very compatible. Neo Mario functions especially well against other Neo Marios, even when you've got four in the same match - they even have some special code to stagger their entrance sounds out so you don't get a mess of noise. Additionally, Neo Mario is capable of bouncing off any character, many projectiles, and every destructible article in the base game (i.e. Kragg rocks, Shovel Knight dirt, etc). Furthermore, he should be able to bounce off of any destructible object you can find on the Workshop. The specific qualifier is that anything which imposes hitstop when it is attacked, as this is the only way I am actually able to detect that Mario has stomped on something without adding specific object-checking compatibility.

This information is primarily for other Workshop creators and describes how Mario's carry affects opponents. If Neo Mario causes problems when he lifts your character, this information may help you figure out why. If not, shoot me a message and I'll help you investigate.
When Neo Mario lifts a character, they are forced into a persistent Landing Lag state. Generally, characters shouldn't be capable of performing any action out of landing lag, so this should hopefully mean that almost any character will be unable to act when being carried. Escaping is done through a special check done by Neo Mario - if the opponent is inputting an up-special and Neo Mario is on the ground, the opponent will be freed and forced to perform an up-special.

There is some special functionality with Forsburn clones - this should not affect your character since if you're setting your characters to be Forsburn clones you're probably doing something you really shouldn't, or you're just on the 9th plane of coding, and either way I'm really scared of you. The special Forsburn clone functionality prevents the carried player from escaping with the previously described up-special escape. Additionally, when Mario is holding a Forsburn clone and he stomps on the clone's owner, it will run some special code to disengage the grab to prevent Mario from holding the clone after it's disappeared (as attacking Forsburn makes his clones disappear). This is a workaround until I implement better checks for characters disappearing while being carried - for the time being, please avoid carrying Forsburn clones in a game with more than 2 players active, as if someone who isn't Mario attacks Forsburn while Mario is holding a clone, things will probably go very wrong.

If your character has issues when Neo Mario grabs them and you don't know why, or if you know of a character who has issues when grabbed by Neo Mario, let me know in this thread.

Grabs have to be given manual support at the moment, but it's quite easy - all I have to do is give Mario the information about the hitbox in a list and he'll detect and react to it automatically. Due to limitations, Mario currently does not respond to grabs from Endless Abyss enemies.
Here is a list of currently supported grabs:
- Ranno's tongue (has special behaviour)
- Clairen's neutral special
- Elliana's uptilt
- Zetterburn's forward tilt
- Etalus's up aerial
- Kirby's neutral special (Copy ability not yet implemented)
Mario escapes from a grab automatically when he is hit with any hitbox that isn't another grab. Additionally, he is automatically freed if the opponent who grabbed him exits the attack state, or if he is allowed to exit the hitstun state, so as to prevent him from getting stuck indefinitely in a grabbed state if an enemy's grab gets interrupted. This shouldn't affect your grabs normally, since a grab is supposed to keep the enemy trapped in hitstun until they're thrown.

If there are other characters from the base cast or Workshop who have grabs, let me know in this thread.

Mario completely ignores Sein's Spirit Flame taps. He takes percent damage, but receives no knockback, no hitpause, and does not lose his power-up. This is because otherwise he would have almost no options against Ori - Spirit Flame can hit before stomp, spin jump, and ground pound.
Mario does not revert by a power-up stage when yoinked by Toon Link's hookshot. However, he still takes percent damage and knockback.

If there are other characters from the base cast or Workshop who you feel need special properties like these on an attack, let me know in this thread.

Neo Mario's victory music will change when he wins on a stage that is considered a castle, or against a character who is considered a boss. Here's a list of boss characters:
- Zetta boss
- Mayu boss
- Godkarmachine o Inary
- Colosso
- Master Hand
- Sans
- Abyss Hime
- King Bowser (Baconator2558)

Here's a list of boss/castle stages:
- Dracula stage
- Eye of Cthulhu stage
- Daroach stage
- Sans boss battle stage (you're out of your MIND if you try to play Neo Mario there though)
- Rezor's Wheel
- King Bowser's Castle (Baconator2558)

If there are other characters that you feel should be categorized as bosses, or stages that you think should be categorized as either boss stages or castles, let me know in this thread.

Here is a list of Workshop items where Mario has special behaviour or compatibility:
- Dracula Boss Battle (unique dialogue for Mario, Luigi, Toad, and Toadette)
- Reznor's Wheel stage (Mario dies instantly when exposed to lava)

More compatibility is coming soon. If there's a character, stage, or something else you'd really really like compatibility for, let me know! I'm planning on adding compatibility features for as much as I can. Here are the things I've already got planned. If something you want is missing, leave a comment in this discussion and I'll add it to the backburner :)
- Compatibility with Miiverse stage (background artworks)
- Compatibility with Pokemon Stadium stage (jumbo-screen artworks)
- Compatibility with the Daroach boss battle stage (unique dialogue for Mario, Luigi, Toad, and Toadette)
- Compatibility with Papyrus (special functionality for the famed Blue Attack)
- Additions to boss/castle list: Tinker Knight Boss Battle Stage