If you got his 3rd CG then you got the good ending!
You have to take the cryptozoology class and in that class the teacher will ask you if you want to learn more about a mythological creature, you have to choose the option "bugbear". And then in one of your classes, he'll randomly appear and you get a chance to ask him questions about himself, you have to get 10 strength so you could ask him about his smile, if you'd done both of that i think you'll be able to get his good end! (although his good end doesn't really seem like its good ahaha...)