Seireitsukai no Blade Dance, Magika no Kenshi to Basileus Recommendations
In both mangas, there are people who attend an academy who, can use "magic" and summon "spirits/divas". According to what they summon, they can use a certain elemental that their summoned being is capable of. The protagonist is a guy, who, is special and seemingly has a shrouded past, and is also surrounded by a group of girls, who may have feelings for him. His summoned being is more of the special type (Seirei, being defeated Demon Lord, Magika, it is Solomon). If you've liked one, you may enjoy the other.
-Both MCs are the demon lord
-Both stories are about the MC, who gets stronger together with girls
-Both MCs are getting stronger because they wish to protect the girls (harem).