I still don't really get his attachment with Watase
Like, I get he's idolizing him. But I wouldn't think it'd be venting-to-an-inmate level of emotional connection.
I changed opinion from my last comment a bit, here's my over-analysis for anyone interested:
The breakup with Asami probably lead Naoki to seek emotional refuge in Watase since he had no other outlet, but Naoki is so invested in Watase because it's implied Watase's the reason he became a prison officer in the first place.
In chapter 2, there's a flashback of Naoki at Asami's place, right before he hears the news about Watase. Naoki says he "doesn't want to look for a job" in Japanese, which I interpret as he hasn't decided to become a prison officer at that point.
Naoki becomes aware of his own aimlessness and regret about giving up baseball when he sees Watase, his childhood hero who had lost everything and despite that continued to dedicate his life toward an ultimate goal. Naoki decides to start taking life seriously and becomes a prison officer. This is the basis of his admiration for Watase outside of thinking his revenge was "cool".