High School DxD, Gate: Jieitai Kanochi nite, Kaku Tatakaeri Recommendations
•The main characters in both anime are strong, despite not appearing as one.
•Both characters share similar-ish personalities (Itami from Gate is less perverted than Issei from DXD).
•Similar genres (ecchi, harem, romance, action, etc.).
•Tons of fan service (way more in Highschool DXD).
•Both anime are long (Gate has 2 seasons - 20+ episodes while Highschool DXD has 3 seasons - 30+ episodes).
If you're like me (who enjoys ecchi, harem anime), then this is the perfect one to watch.