Wars: The Staranian Symbiote Saga

Starts at Chapter 49, continuing the story of a boy named Yuyu Mituamurai. Yuyu arrived at the Demonic Realm and discovered a suit of armor. When Yuyu said the name "Gallet", Yuyu and the suit of armor fused into one new being. Yuyu and Gallet seemed formidable until the arrival of Pluler, a hybrid of a demon and a Staranian, an alien race with strange powers. Being bested by Pluler, Yuyu ventures to grow his own skills while discovering who he really is.

Aliens Fighting Isekai Martial Arts Teenagers (13-18)
chebureiku rivals shonen wars