Goshuushou-sama Ninomiya-kun, Rosario to Vampire Recommendations
the two anime's have several things in common, such as supernatural beings, romance, action, and ecchi comedy, with a hint of mystery!
Very obvious similarities. Both have a "monster" theme to it. While Ninomiya-kun has quite a few succumbi/inccumbi, Rosario has a wider array of weird creatures. Also both involve a confusing harem situation, even though the "main" girls are pretty obvious for both. More importantly, both are extremely high on ecchi theme too.