There are a ton of incidents, misunderstanding and stuff like that.
Every time the MC encounter the heroine he is love with another girl, that make the girl notice him because every normal man falls for her and he doesn't even turn a eye.
MC loses the memory of every girl but only a couple get interested in him in the whole story.
The girl is like the daughter or nephew of some alien race princess or beauty and the monsters wanted her back, is what I think happened, it could even be that beauty attracted another alien race, she works are super popular mode/actress when the granpa uses his secret organization to protect her when she doesn't even realize what is happening, she is nt dumb, they simply hide that stuff well enough.
In the end the aliens sends all their forces together, she gets the truth explained and put into a bunker, when she gets up everything is destroyed, we don't see the final fight, it just happens and we look at her fearing the worst and reacting to noise and tremors.
She and the MC got closer before the end and he got feelings for her that he used to power the suit in said final battle, and then as everyone else he forgot, we see him after the battle, living normally, he goes through a few cliche romcom starting moments with a couple beauties but doesn't react as usual (falls for them) and tells his friends that he felt something inside his heart (a picture is worth 1000 kanji), heroine was close and decided to punchconfess to him, and it seems like he more or less recognized her or the feelings didn't disappear fully.
In short it ends with they probably starting the relationship for real.
It wasn't bad, it probably didn't get translated to the end because at the time of release there weren't many scanlations groups around.