Queen Butterfly is serious most of the time, having a low tolerance for anything messy or destructive. In "Royal Pain", she was said to have kicked her husband out of the castle for not cleaning up his messes. Queen Butterfly often regards safety and caution for Star as shown most prominently in Star vs. the Forces of Evil: The Battle for Mewni when they were forced on the run from Toffee.
Being the queen, Queen Butterfly acts as the true ruling monarch of Mewni and always has the well-being of her kingdom in mind, to the point she is unable to express interest in anything else. This often causes her to appear cold towards others. Despite her overbearing strictness, she truly cares for her daughter; after the near loss of the family wand, rather than show any anger or disappointment, she hugs Star, telling her daughter that she's just glad she's safe. Her concern for Star is also demonstrated in "Toffee" when she attacks Toffee for presumably killing Star.
Moon reveals in Star vs. the Forces of Evil: The Battle for Mewni that she used to be a "happy go-lucky girl" just like Star, but after Toffee and his army killed her mother and she inherited the throne, she became serious and mature to protect her kingdom and family. However, she sometimes shows signs of fear and nervousness, such as when meeting Eclipsa for the first time and when she first casts her darkest spell. Her fear of Eclipsa and her possible escape are also on display in "Toffee".
In "Puddle Defender" and "Total Eclipsa the Moon", Moon demonstrates that she is willing to have an open mind about those toward whom she has preconceived misconceptions, such as monsters and Eclipsa. As of "Escape from The Pie Folk", Moon's return of memory has allowed her to let loose and despite still mistrusting Eclipsa, she still tries to make good with the changes, even forgiving Star for handing the kingdom to Eclipsa, saying a queen has to make choices. She thus leaves with River to make a life for themselves, and even helped out homeless Mewman families that disturbed their property and drove her as crazy as River in "Down by the River". But their unstructured lives in "Ghost of Butterfly Castle" lead to her going back to the abandoned Butterfly Castle to get her day planner, despite River saying it's been haunted or cursed by Eclipsa. She finds that the people of Butterfly Village have gone to anarchy since it was abandoned, and finds that Mina Loveberry is protecting the castle from being looted. She had attempted to turn Eclipsa to stone with 'Yada-Yada Berries' and is distraught when Moon refuses to help her start the Solarian Program operated by her mentor Queen Solaria the Monster Carver, which was a dark time in Mewni's history. Regardless she goes back home and lets Eclipsa deal with Mina.
But despite her reluctance, Moon still cannot let go of her mistrust to Eclipsa. This eventually builds up until she decides to team up with the Magic High Commission and Mina Loveberry to dethrone her by restoring the Solarian Warriors, and have Rhombulus free Globgor and take the fall in "Cornonation" to avoid suspicion. In "Pizza Party", Moon reveals everything and offers Eclipsa and her family safety in another dimension in exchange for the kingdom. Eclipsa accepts, but Mina is unwilling to accept this conclusion and aims to execute Eclipsa, seeing her as a traitor to her mother, Queen Solaria. Moon is shocked that she is unable to undo the Solarian Warriors because it could only be undone by the queen they showed their loyalty to, the deceased Queen Solaria. Hekapoo saves them and brings them to the Tavern at the Edge of the Multiverse, where Star is angry at Moon for dooming Mewni. To make up for her mistake, she helps Star destroy magic in "Cleaved" alongside Eclipsa and Meteora, losing her cheek marks in the process.
Role in the series
After turning up missing and rescued from the Pie folk, she would be back by Saetha's Wedding in time to meet Star's new allies, The Shell Lodge Squad, and actually feels bad for Star's feelings about why she gave up the kingdom and didn't like that her royalty was a lie, because he free spirit didn't mesh well with her royal life, especially with how strict she was, and with what she was about to do with her in an immoral place like St. Olga's Reform School for Wayward Princesses, all for nothing, giving Star a quarter-life crisis, leaving her no choice but to agree with her.