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Princess Terririn is a canon character in both the TSWCAtS roleplays and books, made by Proxy. She was among the main characters in both tales in in Outcasts and Royals: Threaded Tales. Her surname is never specified, but it might be "Castellan", as that is her cousin's surname.
Terririn was born in Eltalor to Queen Venus, the eldest of the four princesses, and thus is the heir to the throne. She is close to her sister Alice, but she never had the chance to get to know her other two sisters, Sora and Melody. She is also Munne Servil's girlfriend.
As a child, Terririn was treated much more normally than most princesses were. Her parents made a point that she was no different from the other children: Even though she would one day become queen, she would get no special treatment, she would not be sheltered like Ofeilian royalty often were. Her parents did their best to expose her to the real world and discipline her, even at an early age, so she would learn humility. She did indeed grow to be a very down-to-earth girl, if not somewhat stubborn and rebellious. As her parents wanted her to socialize with children of different social statuses than her and get a feel for how most children are normally raised, they enrolled her in the school in Anima Town rather than getting her a private tutor, under the condition that Terririn tell nobody of her rank.
Predictably, Terririn immediately broke her promise and boasted to everyone on her first day of school that she was the heir to the throne and quickly became practically the most popular girl at her small school. She formed a small clique with her two best friends, Bonnie Pendragon and Mint Valentine. They became something of a little gang of nuisances, notorious for pulling tricks on the teachers, skipping school (sometimes conspiring to evacuate the school for days at a time), and often times blatantly bullying people they didn't agree with (even though they more often than not wanted to stand up to bullies and help the victims, they sometimes went too far with this and ended up picking on the wrong people).
When she was 12 years old, Terririn and her clique noticed Munne Servil being bullied by two other students, and recognized that it was starting to get violent and the then-tiny Munne did not stand a chance. Unable to sit idly and watch the frail girl be beaten, the clique rushed to her side and defended her. The bullies immediately backed off, and Terririn went to make sure Munne was unharmed. She was perfectly fine, though shaken and surprised that the princess had defended the likes of her, a poverty-ridden, weak-minded little girl, whose grades were terrible and would never amount to anything. Terririn laughed and assured the blushing Munne that she never had good grades either and reckoned that the teachers were biased. They became friends almost instantly, and Munne was invited to be a part of their clique.
Terririn's influence helped Munne to become a stronger, more extroverted person, and they got into all kinds of trouble together. A particularly memorable incident was when the clique decided to skip school and explore the Tigress forest, where they encountered both a Tigress named Ursa and a nature spirit named Serah in the same day. The clique also had a strong rivalry with Munne's older cousin, Ether Astrum, and eventually "won". When Bonnie, Mint, and Terririn learned that Munne's father was the captain of the royal guard, they began secretly "borrowing" his weapons to play with. Terririn once remarked to a blushing Munne that she would look cute in armor. When they got caught, Terririn, Bonnie, and Mint weren't allowed to go to Munne's house, until Munne's parents decided a few weeks later that it was a rather cruel punishment since they were her only friends.
When Terririn was about 15 years old, she began to notice that she was given less attention than her sisters. Perhaps it was because of her snarky, un-princess-like attitude, but some people seemed to act as if her sister Alice was the heir to the throne rather than her. A delegate from Ofeilia even mistook Terririn for a servant, and so she developed resentment towards her sisters and grew very jealous of them. She confided in her parents of this, but they brushed it off as her being an angsty teenager and didn't take her seriously.