Personally speaking, I usually use Yacchin as a nickname for her. I used Yacchan on Yatsuhashi before WB&WC is a thing, though.
On another note, have Rumia got ideas for the rest of WB&WC cast?
So Yachie's nickname shall be... Hmm... Uu-chan,The way this is going, I wonder if she'll call me "Yacchan"...Rumia, what's got you troubled? Chii-chan! She really liked it.Well, um... I'm trying to think of nicknames for the new girls.Call me what you want...Naturally what you'd call a yakuza.
Yacchan (やっちゃん) is a gentle, colloquial way of referring to the yakuza. Ei-chan,Kuu-chan.That's what I've got so far. The Importance of Friendship!