1. so all of them are mutants which refers to when they were experimented on as a child and became a "mutant" basically they got enhanced traits. for example ML is extremely strong, hes considered a 1 man army. Major lee can kill a man just by throwing a pebble. but all mutants share a single deformity which is they are all obssesed with something. for dr.seok it's rocks, for major lee it's food, for major yang it's trains. We are not told yet what MLs is because it's spoilers to later
2. The classes are just a way to measure how strong they are. and Dr.seok want ML semen, because ML is S class, super strong and mc want that good good sample and he a researcher so.

I'm nit even halfway through the story but, I find myself confused about some things.
As someone who tend to skip and forget dialogue and panels, I'm just wanted to know some small dumb info :
- what is an obsession trait ?
-Why is their classes (S, A) important ? Like. Why is Dr. Seok need that MLS semen.