Sister Jeronima was born in Spain and was raised by her grandmother. At some point, she joined the government agency, ACRONYM, which was set up to find fairies. She specialized in interrogation, and once got a parrot to reveal the secret code to a criminal's safe. She almost tracked down Opal Koboi when she was disguised as Belinda Zito.
ACRONYM knew that the Fowl's were connected to fairies. When Dalky Island disappeared due to Lazuli, Jeronima moved in with a helicopter. Myles set off an EMP to stop attackers, and she used this as an excuse to capture Myles and Beckett, since the EMP legally could be considered an act of terrorism. She unknowingly took the troll whistleblower too, which caused Lord Teddy Bleedham-Drye to follow her.
Sister Jeronima proceeded to interrogate both Myles and Beckett, but to no avail. She discovered that the twins had lice, and ordered their heads ot be shaved, not knowing that this was part of Myles' plan. After their heads were shaved, she seemingly discovered that the twins tried to switch places. She called them out and set out to interrogate Becket, while Myles eas dragged away. However, she was tricked, and she was with Myles, who managed to handcuff her to the table and steal her phone. Myles, Beckett, and the fairy Lazuli Heights managed to escape and take the troll with them.
Lord Teddy was about to kill her, but she convinced him to let her live by taking her with him. The two of them managed to track down the twins, Lazuli, and the troll and capture them. While flying to the Scilly Isles on a Myishi jet, she tried to eject Teddy from the plane. However, Teddy tricked her, and her own seat was ejected instead. She fell into the ocean, and the Myishi seat automatically took her to land.
She managed to get a team of ACRONYM agents to the Scilly Isles to kill the Twins and the fairies, However, the LEP put her in a time stop. They mindwiped her to forget about the fairies and sent her to Cameroon to help set up a water pump for a poor village.
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