We have 3 evils and 2 neutrals here - the non-good companions, as mentioned.
In act one, Astarion's "Big Dark Secret" is... "I'm a vampire, bleugh!"
Shadowheart's is "I worship Shar"
Wyll's is "I'm pacted to a devil"
Gale's is "I fucked up and have a bomb in my chest"
What's Lae'zel's super dark side / dark secret?
If you do tadpole path, Lae'zel tells you she also mucked up. She did something that she doesn't want anyone to know, but essentially it was akin to treachery and total disgrace. She won't tell you, but she alludes to it as being the reason she is so desperately trying to do something huge to get into Vlaakith's good graces.
My guess is that she was close to being a kith'rak, because she at one point says it should be her flying on a dragon. It is her right, or something like that. Then she did whatever it was, and now things are even worse because she's the epitome of bad in gith eyes. She's infected. In her eyes, she cannot sink any lower now. That's why she so desperately wants the cure and nags you so much. She can't even begin to redeem herself until she is cured.
