Are CD's inherently evil? Are FMNs inherently good?
Absolutely not. The two gods have been at each other's throats for a long time, before they realized their fights were ripping apart reality. So they made a pact to use their soldiers to fight each other only, and that's where FMNs and CDs come in. The two deities are always scrambling for souls, so they just take anything that can be transformed without checking the content first. So a kind person can be a CD, and a very evil one can be a FMN. They are not devils and angels in the classic sense, and often times their designation has little to do with their human moralities.
What do FMNs/CDs wear?
FMN/CDs wear the clothes they died in, without the damage from how they died. They can wear other things, but their default clothes are whatever they died in - not what they were buried in.
Do they have human needs or age?
No, they don't need to eat or sleep, though they can choose to if they wish. When they do sleep, they don't dream, and the only sleep for a few hours at a time. They also don't age, though they can mature mentally and become older in that sense. Essentially their status is preserved from death, the age they died at is what they'll stay.
Do FMNs/CDs remember their past lives?
Yes, they remember everything that happened, including their death. However, FMNs are forbidden from interacting with anyone from their past life or trying to "solve" their previous problems. They are expected to face forward and try to figure out how to solve their problems now rather than trying to repair their old lives. CDs, on the other hand, are encouraged especially to torment the people from their old life.
Do people they've know in the past recognize them in their FMN/CD form?
In the FMNs case, no. They won't even acknowledge they are there. The person's eyes will just slide right over them. The only exception to this is if the person from their past life is a FMN or CD - in that case the person is immune to the magic that makes people forget. For a CD, they are recognized by people who cared for them in order to facilitate torturing them.
Can FMNs and CDs affect each other besides conversion?
FMNs and CDs have a deep and complicated relationship. They are the only things besides their deities that can permanently hurt or even kill one another, and do so somewhat frequently. CDs can kill and condemn an FMN, ripping their soul away from their path of rebirth and turning them over to the dark god. This is besides being able to convert a living FMN as well, a slow and painful process that gets the CD a lot of clout with their god. But pain isn't all they can give each other.
FMNs are in a unique position to save a CD, though this is a very personal and dangerous process. Only a CD can kill an FMN and only an FMN can save a CD, and this process cannot be interfered with by either deity. If an FMN decides that they believe a CD is worth saving from purgatory and honestly believe with all their heart that they would give their life to help the other to live, they can give their flowers to the CD and, if all goes according to plan, both souls will be reincarnated together as humans. However, this process has to be earnest and wanted by both sides, and even then might not work successfully. If the process fails, both souls will be lost to a void where nothing can ever escape.
They can only become human through this process, the soul of a CD is already too blackened for them to become an FMN, so conversion cannot work the other way around. The process of being saved like this is a very rare occurrence, but sometimes such a bond does happen.
Feel free to ask any questions!