Possible mentors/accomplices for even more killing methods, just to be extra fun:
A maniacal "ex-CIA sniper assassin" who the government has been experimenting on with LSD (We did some shady shit for the sake of sticking it to Communism) has run off to Japan or whatever place Yandere Sim seems to take place in, in order to hide from the Reptilians, with whom he believes he is psychically battling every day. For the right price, you can buy drugs to poison, frame, or toy with your rival(s) from him. For an even higher price, he will teach you close-range killing moves and how to use psychokinesis and telepathy to fight off or subjugate the Alien Scum, I.E. any and all Youkai that may be hiding around the school. Of course, you can use your training to directly abuse the students. He lives in the dumpster, and uses his highly honed commando skills to avoid being found by the custodians and garbage man.
A lecherous ghoul who lives in the Ag Club's garden shed and digs up the bodies of girls left behind by previous Yanderes. He will devour evidence for you in exchange for used panties, body pillows, and other such perverted anime shit, on the condition that you let him keep any remaining fuckable bits buried in jars somewhere off campus for preservation and/or pickling. If you triple-dog dare a student to come to school at night and serenade the shed, (assuming you gave the ghoul that girl's panties first, so he can be brought out by the familiar smell) he will maul and fuck her dead corpse before eating it.
A shady-looking girl in a torn uniform, the ghost of one of the previous Yanderes and/or one of the ghoul's victims, sits on an opened, flattened cardboard box in a small clearing of branches hidden between hedges. She is a gambling addict, and will play dice with you for free. Assuming you beat her three times, she will make... Bigger bets with you. If you can kill certain student(s) in certain amounts of time a certain way, she'll destroy all the evidence and drag your rival to Hell. If you can't, then you go to Hell instead. (Well, you go to Hell prematurely, I guess.)
A pale, creepy girl who sits in the back of each class reading thick, leather tomes of history, all-too-intrigued by the most brutal people of the Medieval ages. She would do anything, anything, to see someone being executed in one of the old fashioned ways, pulled apart by running horses, impaled on giant stakes, drawn and quartered, boiled alive, crucified, etc. but she only has the time and resources to stab somebody. She'll kill your rival, confess to both crimes, and go to the chair happily if you show her a good, authentic reenactment.