Agnes is an extreme introvert who has a giant love of magic, specially dark magic. As a princess she was never much outgoing and would attend public events only due to her mother's insistence, she preffered staying in her room where she kept lots of magic books from the library (used the castle's secret passages a lot to sneak there). In spite of that Agnes is very found of make up and would spend a lot of time in her daily makeup.

In close match with her love of books, Agnes also loved animals. Her beloved pet cat, Nori, could almost always be seen near her, sleeping, purring or just wandering around. Her wand even copied its ears and she made lots of spells to play with Nori. Agnes also loved singing and most of her spells are done by softly chanting the verses.

As a child, Agnes was very shy and never showed too much, she would usually be behind her mother, running away from any attention she got. As she grew older she realized she could not run away from her responsabilities as future queen and started to accept the attention although still being reserved.

After she got her wand Agnes dive straight into the magic world, spending most of her free time researching everything she could of it. Not long after she found out about the dark magic and soon would suffer the consequences from it. Her arm became blackened from the use of dark magic but it wouldnt stop her from researching it. At one point, her mother, worried with her, banished Agnes from the library. That only prompted the girl to find out about the castle's secret passages and used it to reach the library.

Soon, with Glossaryck's help, Agnes was able to dip down and to pass her mewberty. She had a fairly calm life untill she was 17, when her pet cat Nori died. Agnes could not, and would not, accept it. She did the biggest research of her life and, against Glossaryck's advice, created and cast a spell to revive the dead. The spell was cast in the royal cemetery Nori was buried in but it was so strong it reached every Mewni land around and consumed Agnes left eye, leaving a black hole behind.

Nori was succesfully revived and seemed to be the same pet as before. Not soon after, however, the mewnians would start to witness their loved ones standing again, but not so well. An army of undead would be always at Mewni's doorstep, looking to destroy anything that lived. Defeating these beings seemed to be of no help since they would just get up right after and chase others down again. It was the start of the "Age of the Walls" in Mewni, in wich a giant wall would be lift around the city to keep the undead away. Agnes would gain the title of "The Necromancer" as she tried to fight to keep the undead away while diving into books trying to find a solution for the problem. Until one day Musica was born. They didnt have a much close relantionship, because of Agnes uneding research, but she tried her best to be around Musica. She would always keep her around while researching and sang lullabies for her.

Unfortunately, Agnes died at the age of ???, never finding a solution to the undead spell. Unable to be buried, her ashes were kept in a urn that Nori, still undead, knocked to the ground.