Zero no Tsukaima, Rosario to Vampire Recommendations
Both are ecchi animes that have similar characters and are set at a mystical school. ZnT is set at a magic school, RV at a monster one. Plus, in both animes, the main girl has pink hair, and the main boys in both are similar. They both have Rie Kugimiya too!
You have a run-of-the-mill human wind up in a school for crazy magical folks, and he has to figure out how to survive. Meanwhile, the harem builds up around them, and then slapstick abounds.
both guys get plunged into some weird and magical world.
there both very helpless because of the distance between the human (guys) and these other people and then they try to close this gap.
Both animes feature charming female leads, magic, and school life. Zero, sadly, has a low level of fanservice compared to Rosario, but a better story. However, Saito is way cooler than the loser Tsukune so that is an advantage that Zero has over Rosario.
Both harem, fantasy, romance, and are set in two worlds. If you like one you should like the other.
Both male protagonists went from the normal human world to another dimension/planet. Also some characters are similar to each other.