Diyann is one of the Jhereg Council, of which there are around six.
Diyann was one of the three Council member's who attended the meeting with the renegade Easterner, Vladimir Taltos, to review a new business the latter was presenting.
Spoiler warning: all information disclosed about Diyann is appearance
Nothing is known about Diyann other than his appearance, which is that of a pleasant landlord who lives eternally in harmony with the world. His appearance is a lie.