This manga is really difficult to rate. It's like a completely different story starting at volume 4, a not very interesting one, but until that point it was really good.

From the beginning of the manga it gives a strong sense that Narumi is the protagonist. He is quirky, interesting, and pulls all the action and gives energy to the story. The kid who is supposedly the protagonist is generic, not very interesting, and gets less screen time. But at the end of volume 3 it changes.

What this means is, the manga starts ok, becomes very captivating, engaging, and excellent, with the culmination point at the end of volume 3. After that point the manga becomes entirely generic, boring, and looses all energy. It feels like it suddenly turned from an engaging dramatic fighting manga full of action into almost a typical slice-of-life school manga about a kid, with the kid as the protagonist, but not an interesting protagonist. There is also the girl but her character (or more like, her lack of character) fails to keep the story interesting.
Until volume 4 I became very engaged and then it stopped, I read the 4'th volume but it just wasn't very interesting in comparison. The action was just kind of going slowly and without a point.

If I had to rate the first 3 volumes I would say it's a 9/10
Volume 4 was a 5/10 and I dropped it.
Because of this, I find it difficult to give it an overall rating. I would rather not rate it at all, but if I must it will be an average of 5 and 9, that is 7

I think you won't loose much from reading only the first 3 volumes, because it's like a complete story of its own. So feel free to try it out.