It must be the little man who used to work for Hirata and turned coats.. there were two of them remember?
they were the ones driving the car and going to Yashiro's But Doumeki hit him and took his place..
Here:
http://www.mangago.zone/read-manga/saezuru_tori_wa_habatakanai/mf/6ba770cf8927a77ff194ed50229a1153_chapter-15/pg-8/
it's this page and the ones after it..
Well, when Nanahara saw Yashiro making that ohone call he protested saying that Yashiro was going to run on his own and try to solve things all by himself...I suppose Yashiro either beat him for his impertinence or because he didn't want to put him in danger by taking him along.
As for where Yashiro is going, I don't really know.. I guess we'll find out in the next chapter..
Chestnut is part of a team of Hirata's mercenary muscle. Chestnut doesn't have a name, but when we see him he's always eating a bag a chestnuts. So Yashiro calls him Chestnut.
After he beat up Nanahara and left Yashiro got on the phone and expected someone he called Chestnut. Who do you think he called?