Seikon No Qwaser is an mature themed dark gothic story that features a pair of highly demented twins (named Q and R), who enjoy pain and misery because they were broken that way by their “mother”. They go yandere on people who DON’T inflict pain and misery on them… but if you do, they love you lots and lots. The pair even carry bags of torture devices to use on them. Of course when you run out of different ways to abuse them - they possess the power of instant regeneration, so they can go on forever - you’re dead. Obviously on Team Evil, and only last for a single story arc, but fun twincest.