Arizza was the only human slave living in the Xanathar's lair, as it preferred elven slaves. Arizza was an exception because of her eidetic memory, which made her the perfect bookkeeper for the beholder. She was always at the Xanathar's side during business meetings, ready to use her memory.
Arizza, thanks to her memory, was very good with numbers and figures. She had few social skills. Arizza would never betray the Xanathar or do something that would hurt him.
Arizza, having no family or friends, had developed an attachment to the Xanathar.