could someone please explain the story up to this point

I was absolutely loving this game but got really stuck on Seymours first fight so I took a couple months off. Ive recently come back and just beat him (nothing more) but I really don't remember the finer details of the story so could you just go over the important things that have happened so far leading up to finding out Seymours bad. Also is finding Seymour is bad meant to be a huge plot twist or something or do most people already know because of him being a big boss fight(s). Also the reason I was really stuck on Seymour was because I was pretty damn under levelled so im probably going to grind for bit, is there a reference point of how levelled I should be before I carry on ie should X person have Y ability. That's about it , thank you!