Confidence (自信, Jishin) is an extra chapter included with Volume 17.
Following on from Speed-o'-Sound Sonic's decision to eat a Monster Cell, Sonic has diarrhea for the whole night. After the ordeal, Sonic believes he has become a monster and that while, he hasn't changed his appearance, his body has undergone a massive change. He has vindictive thoughts about Gale Wind and Hellfire Flame for not telling Sonic about the diarrhea. However he puts the thought aside as he never planned to join the Monster Association, he feels great power within himself. Feeling lighter and more powerful, Sonic is brimming with confidence and states he will need time to adjust to his new body. In reality, Sonic just lost 2 kg from his diarrhea and is mistaking his condition as his new monster form.