Who's "him"?

Who exactly is "HIM" if you don't want to post HIM's Name then PM me HIM's Name

man, this stuff it's pretty crazy

Allow me to sum up everything I've found, of course this is thanks to everyone who's posted their findings before me as well.

Sandbag - Clear Classic as Random
Mushroom Kingdom 64 - Clear Classic for the first time
Metal Cavern - Clear Classic on Hard
Hyrule Castle 64 - Clear Classic as Link on Normal
Saffron City 64 - Clear Classic as Jigglypuff on Normal
World Tournament - Play 10 vs Matches
Waiting Room - Play 1 vs Match as Sandbag
Devil's Machine - SD 3 times as Ness with Ghost skin
Steel Diver - Travel a certain distance underwater (Not sure what distance exactly, I got by SDing 25 times on Lake of Rage)
Final Valley - Clear Event 12
Desk - Clear Event 26
Alternate Music Tracks - Open the game a certain number of times (This one might be random, but that's how it happened for me)

There might still be more, such as Distant Planet and Shadow Moses Island assuming they haven't been removed.

I got devil's machine by SDing 3 times then quitting but other than that it looks good.

Alright, I edited it. Let me know if any of these are wrong and I'll correct them.

steel diver has to do with underwater time, deaths have nothing to do with it

So I've A/S ranked every event up to and including 51 yet nothing unlocked aside from a notice. It seems unlikely that they'd hide content dependent on achieving S-ranks in every event, (given fewer people would be able to accomplish this) but can anyone confirm whether or not this the case?

Did anyone try to see if there are any unlocks based on Target Smash, Home-Run Contest and Multi Man Smash?
Level 1 of Target Smash did not seem to unlock anything although I'm not sure about the rest.

That makes a lot more sense, I'll edit it. Though not sure exactly how much time,

Saffron City cannot be unlocked by Pikachu, only can be by Jiggly

But I cleared it (E rank) and I got the Sandbag unlock notice (even tho I already had him)

Check the thread, it's bugged right now. It shows the message for Sandbag but it actually unlocks the Desk Stage.

Oh, I see it now, thanks

What I have found:

1-6: Stages
7: Secret Character

1. "Mushroom Kingdom [64]": Beat Classic mode on any difficulty for the first time, or play Group Smash a few times to unlock this stage.
2. "World Tournament": Play Group Smash a few times to unlock this stage.
3. "Final Valley": Beat the 12th event in Event Mode to unlock this stage.
4. "Desk": Beat the 26th event in Event Mode to unlock this stage. (When completed, the game glitches and shows that you have unlocked the secret character, Sandbag, but you get the "Desk" stage instead.)
5. "Devil's Machine": In Group Smash, set the stock counter to 4 stocks and then choose the character, Ness, and pick his alternate skin that makes him look like a ghost. Go to any stage and when the battle starts, jump off the stage 3 times. Beat the CPU with 1 stock remaining to unlock this stage. (CPU level doesn't matter.)
6. "Waiting Room": Play a match of Group Smash with the secret character, Sandbag, to unlock this stage.
7. "Sandbag": Beat Classic Mode with a random character on Normal difficulty or higher without using a continue (using all lives and pressing continue). Afterwards, you will be sent off to fight the secret character, Sandbag, in the secret stage, "Waiting Room". Beat Sandbag to unlock him.

I hope some of you found this to be helpful!