Slasher’s Keep Character Unlocks Guide

Though I marked them together, you can’t get both at the same time.

You unlock both, one at a time, from finishing the final boss.
There are generally two ways of approaching him: the hard way, and the easy way.

Sasquatch
Doing it the hard way gives you this character. How? Just beat the VERY BEEFY melee boss, the traditional overpower way or knocking em to the spikes available on the map. Haven’t really tried the spikes cause it seems like a cop-out of a final boss, so I spent half an hour just whacking away.
After the melee boss lies dead, there should be a dropped monkey paw. Pick it up to unlock the Sasquatch, then pick the golden key, and claim your freedom!

Undead
The easy way nets you the undead. How? Climb up 10 floors again, but this time kill the magic boss without killing the melee boss, because if you’ve unlocked Sasquatch you would know that the magic boss runs away the moment melee boss dies.
Hint: wands are useless against the magic boss. The only way is (spoiler alert for those who wants to figure it out themselves): Destroy the columns of the balcony he is on, kind of like how you use the barrel trap on the first biome. Except the columns are pretty durable, so prepare lots of invulnerability potion so you can whack at ease, or just kite melee boss while you’re at it.
When you’re done, there should be a jade monkey statue. Pick it up to unlock the Undead. Do what you will with the melee boss afterwards, I generally help them.

It’s me, ya boi.
He’s a pretty straightforward lad, just find a unique red shrine somewhere after the third floor of a non-starter dungeon (meaning any dungeon after you’ve escaped Slasher’s Keep the first time) that spawns pretty rarely. Sacrifice your flesh prison and embrace true darkness, unlocking the character. Simple, but annoying, because it’s pretty damn rare.

This blob’s the most annoying of them all, not because of how hard it is, but because how much time you’ll probably need to sacrifice before you can get him.
Just pray RNGesus is on your side, and don’t expect you’ll get him/her/it before you finish your first escape.To unlock, you need to do three simple things:
1. Find an “energized slime sample”
2. Find an “energized acid sample”
3. Get to the third biome, clear every room to find an alchemy apparatus, and insert the two items to get a potion. Drink the potion. Have your bones melt away and transform yourself into a whole different character entirely (might wanna save/reload/think twice).

The problem?
Both samples are excruciatingly rare. The slime sample drops from blue slimes, and the acid sample drops from – you guessed it – acid slimes, but both have like, idk, 1 in 1000 drop chance? It took me awhile before I got both, which brings us to the final problem:
the alchemy appartus only spawns on the third biome, and can spawn at any floor of said biome. You have no choice but to keep these inventory-consuming items on you at all times, without dying, clearing every room in the hopes of seeing the thing. I personally haven’t unlocked this guy.
Well, not exactly that you have no choice. I don’t know exactly the logistics, but earlier in the run I found only the slime sample, which I sent down the chute. I found the alchemy set in that run, and while interacting with it, the clearly-in-chute sample suddenly popped up and appeared in the alchemy set. Idk if that’s bugged, intended, or patched out, but something to try if you got unlucky and already chuted one sample when the pair appears.