Unfortunately due to limits in the API the mod can't automatically detect what costumes a character starts with and they have to be set manually, for the vanilla characters this was easy enough, but modded characters are different.

I've set up the mod to be able to create a list of modded character starting costumes to ignore, which you can add the costumes to through a few different methods, though to do any of them you need to know the file path of the costume you want to add.

To find out what you need to enter for a modded costume you just need to go to the mods folder (e.g. Users\NAME\Documents\my games\Binding of Isaac Afterbirth+ Mods\revelations_1536643474 ) then go into 'content' and open the 'costumes2.xml' file, find the costume you want to add and take its path (e.g. 'characters/revel2/character_dante.anm2'), then take the 'anm2root' from the top of the file (e.g. "gfx/") and add it to the start of the costume path.

Once you have the file path of the costume you can do one of the following.

A: (for modders) add a function into a mods lua in the playerinit or gamestart callback
e.g. if NoCostumesMod then
addCostumeToIgnoreList("gfx/characters/revel2/character_dante.anm2")
end

B: open the console by pressing ~ and enter a command
e.g. addcostumetoignorelist gfx/characters/revel2/character_dante.anm2

C: paste the costumes file path into the mods save file
you can find the file by going to 'Users\NAME\Documents\my games\Binding of Isaac Afterbirth+ Mods\nocostumes_1590327567' and it should be a .dat file

Make sure not to remove the number at the start of the file.

When adding costumes to the save file you just need to make sure that you add a ' before the file path and a - after it, you can also include mod names by using a couple dashes
Example text below


--revelations character costumes
'gfx/characters/revel2/character_dante.anm2-
'gfx/characters/revel2/character_dante_hair.anm2-
'gfx/characters/revel2/character_charon_hair.anm2-
'gfx/characters/revel2/item_brokenoar.anm2-

--chronos character costumes
'animations/costumes/chronos_ears.anm2-
'animations/costumes/chronos_horn.anm2-

--mei character costume
'gfx/characters/meisheavyblackhair.anm2-

--gaia character costume
'gfx/characters/gaia_hair.anm2-

--moth character costume
'gfx/characters/character_mothhair.anm2-

--ness character costume
'gfx/characters/ness_hat.anm2-

--inkling character costumes
'gfx/characters/Inkling_hair_Green.anm2-
'gfx/characters/Inkling_hair_Cyan.anm2-
'gfx/characters/Inkling_hair_Blue.anm2-
'gfx/characters/Inkling_hair_Purple.anm2-
'gfx/characters/Inkling_hair_Pink.anm2-
'gfx/characters/Inkling_hair_Red.anm2-
'gfx/characters/Inkling_hair_Orange.anm2-
'gfx/characters/Inkling_hair_Yellow.anm2-

--octoling character costumes
'gfx/characters/Octoling_hair_Green.anm2-
'gfx/characters/Octoling_hair_Cyan.anm2-
'gfx/characters/Octoling_hair_Blue.anm2-
'gfx/characters/Octoling_hair_Purple.anm2-
'gfx/characters/Octoling_hair_Pink.anm2-
'gfx/characters/Octoling_hair_Red.anm2-
'gfx/characters/Octoling_hair_Orange.anm2-
'gfx/characters/Octoling_hair_Yellow.anm2-

--doki doki club costumes
'gfx/characters/monikahair.anm2-
'gfx/characters/sayorihair.anm2-
'gfx/characters/natsukihair.anm2-
'gfx/characters/yurihair.anm2-
'gfx/characters/razorbladeyuri.anm2-
'gfx/characters/ACT2.anm2-
'gfx/characters/sayonara.anm2-
'gfx/characters/sayonarahurt.anm2-
'gfx/characters/sayonara2.anm2-
'gfx/characters/sayonarahurt2.anm2-
'gfx/characters/playwithme.anm2-
'gfx/characters/justmonika.anm2-
'gfx/characters/sayoriglow.anm2-
'gfx/characters/natsukiflight.anm2-
'gfx/characters/yuriflight.anm2-
'gfx/characters/monikaflight.anm2-
'gfx/characters/onlymonika.anm2-
'gfx/characters/hypergraphia.anm2-
'gfx/characters/amy_likes_spiders.anm2-
'gfx/characters/cryptic_poem.anm2-
'gfx/characters/dear_sunshine.anm2-
'gfx/characters/portrait_of_markov.anm2-



Feel free to post more here so others can just copy paste.