This guide is here to show you what the conditions for Character Unlocks, Alternate Loadouts and Character Costumes are. I tried to be as non-spoilery in the main body as possible, the Glossary + Notes has some spoilers as I try to clarify a few things.

In order to unlock the ability to play as one of the Bosses, you must first beat them when they are at Tier 3 or higher. Or in other words, beat the Boss at 4-7 or Higher (This will be the fourth zone you go through).

Reva can also be unlocked upon reaching Profile lvl 6.

The Shopkeeper is unlocked by beating her in combat

Every Character except the Shopkeeper has at least one additional starting Loadout. These change the character's starting cards, artifact and weapon.

Saffron's Chrono Loadout - Unlocked by reaching Profile Lvl 2
Saffron's Solo Loadout - Unlocked by completing a Full Genocide Run including the Shopkeeper as Saffron

Reva's Beat Loadout - Unlocked by completing a Neutral or Genocide Run as Reva

Gunner's Bullet Hell Loadout - Unlocked by completing a Neutral or Genocide Run as Gunner

Selicy's Invade Loadout - Unlocked by completing a Neutral or Genocide Run as Selicy

Hazel's Teardown Loadout - Unlocked by completing a Neutral or Genocide Run as Hazel

Terra's Pyro Loadout - Unlocked by completing a Neutral or Genocide Run as Terra

Shiso's Kunai Loadout - Unlocked by completing a Neutral or Genocide Run as Shiso

Violette's Aria Loadout - Unlocked by completing a Neutral or Genocide Run as Violette

Saffron Legacy Skin - Unlocked by getting the Arch Sage achievement or by completing a Pacifist run as Saffron.

Saffron Lea Skin - Unlocked by getting the Calamity achievement or by completing a Pacifist run as Saffron.

Saffron Witch Skin - Unlocked by completing a Pacifist run as Saffron.

Reva Cyber Skin - Unlocked by getting the The Best Defense achievement or by completing a Pacifist run as Reva.

Hazel Priestess Skin - Unlocked by getting the Absolute Zero achievement or by completing a Pacifist run as Hazel.

Selicy Catgirl Skin - Unlocked by getting the Knife to a Gunfight achievement or by completing a Pacifist run as Selicy.

Gunner Buccaneer Skin - Unlocked by getting the Solo achievement or by completing a Pacifist run as Gunner.

Terra Dreadwyrm Skin - Unlocked by getting the Terraform achievement or by completing a Pacifist run as Terra.

Shiso Cowboy Skin - Unlocked by getting the Not Even Close Baby achievement or by completing a Pacifist run as Shiso.

Violette J-Rock Skin - Unlocked by getting the Buzzer Beater achievement or by completing a Pacifist run as Violette.

Shopkeeper Legacy Skin - Unlocked by completing a Pacifist run as Shopkeeper. V1.2 (V1.1 hotfix?).

This section contains spoilers! Proceed with Caution.

Pacifist runs - Spare all the Bosses

Note:It doesn't seem to matter if you Spare or Execute the final boss for Skin unlocks

Neutral runs - Execute at least 1 Boss, but not all of them Note: All that matters is that you destroy the Gate, doing so counts as a "victory"

Genocide runs - Kill all Bosses, but not the Shopkeeper Note: Even if you get Looped it will count as a victory if you die during the loop

Full Genocide runs - Kill all bosses, including the Shopkeeper Note: Make sure you Execute the final boss

If you notice any incorrect and or missing info here please leave a comment!