User blog comment:DoomFest/~The Great Thumbnail Contest - Season 2 ~/@comment-29048763-20160410171231/@comment-29048763-20160410181956

Both are practically overpowered heroes who are not known by the human public for their deeds but do it anyway for fun and also to make sure to keep up with a contract otherwise something terrible happens to them (Being thrown out of the Heroes association for Saitama and having her soul taken down to hell by the demons for Bayonetta). They also hate a certain type of insect, Mosquitos for Saitama and Cockroaches for Bayonetta. (Hence the bottom pic teaming them up) and finally...it's a bald guy taking on a witch who uses her hair as clothin and summoning demons. You can tell Saitama will take this fight personally because of that.