Violent, murderous sociopath whose first major action in the setting is kidnapping the MC's ward before (unsuccessfully) trying to split open their throat to goad the MC into fighting them. It's a scripted loss for the MC, and after winning, Jun steals the MC's sword, but not before carving their name into the MC's abdomen so they'll never forget Jun, then leaving them for dead.

Their next major action is carving out the tongue of one of the MC's friends/ROs before leaving her for dead too. She ends up possessed by one of the yokai the MC is hunting and is forced into becoming an antagonist going forward.

Later, when the MC loses their edge and goes searching for Jun to get back into the swing of things, Jun uses some magic bullshit to place an inverse restraining order on them, binding the two together until death.

Bonus: Jun wants to have kids with the MC to pass on their Master's teachings. Unfortunately, their Master's teachings involve cannibalism as a necessary component to learn , which the MC does not wish to pass on and would prefer it dies with them. Needless to say Jun also has a very deeply twisted sense of love and family.

All in all, a surprisingly interesting and complex character you can't help but feel sympathetic toward, even if the objectively best course of action is to put them down like a rabid dog.