The youngest member of the extended Smith-Rhodes family to be introduced so far, Mariella is eleven when first seen, as the youngest child of Andreas and Agnetha Smith-Rhodes and youngest sister of Johanna Smith-Rhodes. She is seriously minded, quiet and determined to see the world outside Howondaland, and also admires and adores her oldest sister who found the way out. Mariella has won one of the coveted places at the Assassins' Guild School for her year of entry, against tough competition in a national contest. Her family name helped, as did the fact her older sister is a starred guild graduate. Assassins respect this sort of thing, and family head Charles Smith-Rhodes sees nothing wrong in another member of the family getting a Guild education. Mariella does not know this yet, nor does she know who is paying her school fees.

At the School, she discovers an aptitude for writing that leads her to the School newsletter, the Cloak and Dagger. Both Sacharissa Cripslock and her journalist cousin Suki van der Graaf are firmly of the opinion that she has a useful talent. She is also a founder member of the School club, the R.A.T.S. The RATS (Relatives of Assassin Teachers and Staff) is an association of Guild School pupils who either currently are related to members of staff, or like the unfortunate Margaret Band (same name, no relation) are mistakenly thought to be.

She is a formidable fighter when angered, and her ability to complete a tough cross-country run usually in first place without even breathing heavily is something that alarms the PE staff. Her aptitude for observational writing is a talent which senior staff members consider it wise to monitor. Her older sister acts as her next-of-kin and guardian while in Ankh-Morpork.

She appears in:

The Black Sheep (introduced in final chapter)

The Prospectus, as one of the selected students providing testimonials as to why the Assassins' Guild School is a great place to live and learn.

Hyperemesis Gravidarum sees more of Mariella as a character, a soon-to-be auntie who discovers that training to be an Assassin can be dangerous.

In The King of Swords she learns more about her humble family origins in the village of Scrote.

Gap Year Adventures sees her touring a continent as a graduate Assassin with her best friend Rivka ben-Divorah.