First of all, he is hilarious and the total mood maker of the group. Probably the best way we’ve seen Nagisa’s humour is through Rei.
I mean just imagine episode 6 without Nagisa. It would have been so tense and totally not fun with Haru being irritated at Rei and Makoto not smilng :(
There’s also how the whole reason Rei is on the swim club is because of Nagisa’s assertive and unique tactics.
Nagisa’s also quite aware of his friends and pretty insightful.
When Haru was throwing a hissyfit about not being able to swim right away, Nagisa immediately sensed the tension and was quick to moderate the situation.
When Haru is about to give Rei a piece of his mind for swimming by himself late at night, Nagisa chimes in and explains Rei’s thoughts on behalf of Rei, despite no previous discussion about it.
Makoto doesn’t smile after the whole ocean incident, and is (as Nagisa describes it) “subdued”. Nagisa does his awful rockhopper penguin impression and finally gets a smile out of Makoto. :)
When Haru isolates himself after his loss, Nagisa can’t stand it. Nagisa figures that Haru must be there to support his friends so he rushes off to grab Haru, which was definitely beneficial to not only Haru, but to the rest of the team as well.
Nagisa is probably the first character to really take notice of the nature and strangeness of Rin’s effect on Haru.
Not to mention that Nagisa doesn’t hold a grudge against Rin at all, even after Rin being a pretty big jerk towards him and Makoto in episode 1. He still calls Rin “Rin-chan”. <3
Bonus (High Speed!): We can probably count the amount of times on one hand that Haru has smiled in the entire plot so far. One of those times is in this moment between Haru and Nagisa.
Honestly, everybody should have a friend like Nagisa.