The main character is a man/boy around 16-20 years old, the story takes place in a city, and is about fighting monsters, criminals, aliens. I know that he equips a helmet and transforms to battle them, the transformation is similar to Iron Man, and the helmet looks like a motorcycle one with 2 horns (one of which is broken), I think.
I remember one episode where he is fighting one monster/alien on the roof of a building and loses the battle, until he absorbs an antenna in his arm, then pulls out a sword from that arm.
I saw this anime on TV maybe 8-9 years ago.