He's a skill trainer in Sayuka that gives insight and intimidate as well as a powerful necklace item if you pass his skill checks. If you receive the necklace first and try to train with him afterwards he begrudgingly says yes but the only option that appears is to decline him. However, if you train with him first you can receive his necklace without issue.You just have to reload and talk to him again in the correct order but some people might miss it.
I did a completionist run of Pillars1 on story mode and it took me around 60 hours listening/reading all the dialogue. Was worth it.