Shemeshka is a fiend, an arcanaloth that lives in the planar city of Sigil, who goes by the name "King of the Crosstrade". Long ago, she tried to buy Strix, who was a child at the time and was living with her mentor, Baba Yaga.
Whenever Diath puts a key into his sword, Gutter, she appears. She will answer three questions in exchange for one of the keys. Once Diath agreed to a geas spell, she teleported the Wafflecrew to Port Nyanzaru and gave them the means to summon the Sewn Sisters.  When Diath was in danger of being caught in a massive explosion, she cast banishment on Diath, causing him to appear at a party in Sigil with her. He demanded to be brought back, and ultimately she relented. 
Widow Groat told the group that, as payment for drafting the Ashton Concordance, Shemeshka has a piece of Diath's soul. She keeps it in a box in her house. She also explained that Mr. Fox and Shemeshka are enemies, and that her truename is kept in one of the Books of Keeping.