It is a nonspecific sin type, that takes the form of an animal unless you use a power. Its capabilities without using your powers to enhance it are CAT 0. It does not have any stats, instead using your skills. It cannot have sin abilities. If you are bad at fighting there are one or two BIND abilities that will still give your sin an edge in combat, but it costs a psyche burst and isnt permanent. Your passive does actually specify that you have a single bound sin at the start of the game, and has no actual rules text discussing binding additional sins (likely because it would seriously fuck up game flow to allow exorcists to bind sins instead of executing them at the end of a hunt).
There is a homebrew project in the works to revamp binding, I am not sure the progress on it. I have played with a binder in my party and its really not that bad as is, and the rules are pretty clear in my opinion. It lacks the Fushiguro flavor that I think it deserves, but as is it is functional enough for me to use and flexible enough for me to make on the spot rulings and homebrew new ideas.