We'll start with some basic facts that you need to know before continuing.
In this lore, soul bonds are a huge deal. A soul bond represents the possible or current relationship that one person has to another. Golden soul bonds represent soulmates. Silver represents high compatibility. And so on and so forth.
People are meant to have hundreds of soul bonds. Think, everyone you have ever interacted with on any level you have a soul bond with. Therefore, people should have hundreds (if not thousands). Some weaker ones fade in a matter of weeks or years (you aren't going to have an intense soul bond with your waitress you had at Denny's while on a road trip. That bond will fade in a few weeks). However, potential soul bonds appear when people are going to possibly meet. These soul bonds are weak at first, and have potential to grow or fade.
A soulmate can be a brother or a friend, but can also be a lover. And soulmates do not have to get along; people who are soulmates can hate each other and ruin the bond.
Reincarnation is a thing. Not mentioned at all, I don't think, but it is a thing.
Spirits and gods are also things. Most relevant are nature spirits and death gods. A nature spirit embodies something; an animal, a plant. They get a few abilities in addition to this, the main one being the ability to turn into the thing they are embodying. Animal spirits have more freedom than plant spirits and can be limited in other ways, but a plant spirit is happy with their sedentary lifestyle and can move if need be. Captain is a fox spirit. He has abilities related to mischief, like invisibility and floating (that's just laziness). His species mate to each other, like many other animal nature spirits do. His species mates with a bite of intention; if done at the right moment (during a mutual orgasm) by both partners, the two will be mated. This can be done with any species, as long as one is a fox. Captain's mate mark from Henry is one with the intention of domination. Therefore, their relationship is reliant on Henry's control and Captain's submissiveness.
Death gods are created from spirits who chose suicide over a natural death. By implying they had control over death, they offend her, and the punishment is to do her dirty work. (In reality, she's also just kinda lazy and hates her job. It makes her sad.)
Death gods are trained at Academy's. There are a few located in central places for spirits to go to, graduate, and disperse from. If a spirit fails, they can try again until they succeed, or they can accept failure and wander the world for eternity. The danger in wandering is other, stronger spirits (and things like demons) that pray on the defenseless. Once trained, death gods can either work in the field or work at an Academy. The upside of working in the Academy is a cushy job with no danger. The downside is that one must be very talented to land a job training others, and to get to this level of talent requires a long time in the field.
Once a death god completes their task, (a number of souls reaped, a number of years served) they can move on to the afterlife, or retire without their abilities. Very few retire.
Death gods are not meant to leave their jobs with their abilities. Few have attempted and succeeded.
Madsie and Madden are the same person, in some ways. Parker and Four+BB are the same person. Madden was around before Madsie. His story was known first. When Madsie split off from him, Madden kept his own story and Madsie got an alternate version of it. Parker experienced the loss of a friend (who blamed him angrily and then left). He split into Four+BB as a result. They have a different backstory as they're different people. Kinda the same, because if you smoosh them together you get Parker, but kind of different from him.