Monica was first introduced in Aylee (chapter) as Riff's girlfriend.
We discover in Mandatory Applause that Monica has a tattoo on her stomach of the Chilus symbol, a cult of K'Z'K. She is helping the cult spy on Riff, Gwynn, and Zoë.
In BROKEN (specifically Roken), Oasis stabs Monica for wearing Hereti-Corp clothing from the House of Cheese (link).
It turns out that Reynold Strom had been following Monica, and after her death he learns that her real name is Erin, and seems interested in finding out more (link).