Nina is serious, determined and hard working because she wants to win the love of her adoptive father Sergay. Nina holds mixed feelings towards Arika Yumemiya. She is jealous of Arika's connections with Sergay and often annoyed with her cluelessness, but she shows friendship towards her on occasion. Nina's friend, Erstin Ho, was also her roommate who was in love with her. This was most evident during episode 6 where Erstin performed CPR on Nina in a rather affectionate way, leaving Nina embarrassed.
As time passes, Nina begins to loosen up due to Arika who is the complete opposite of her and begins to respect her as a good friend and rival. However, she started to notice Sergay and Arika being affectionate towards each other and begins to break down slowly as Sergay, her father, is her number one person in the world. This began when Arika called her father by his first name, Sergay, which leaves her shocked and upset. Later on, Sergay sends Nina a birthday present as she turns 15 but when Arika reveals that it was also her birthday, Nina receives yet another shock. When she was young, Nina found out that Sergay, as a boy attending school in Windbloom, had a major crush on Rena, Arika's mother. Sergay also made the 7th of September Nina's birthday as she didn't know, ironically, the 7th was also Princess Mashiro Blan de Windbloom's birthday. Nevertheless, she believes that the reason why Sergay gave her the date was because of Rena. When Arika addresses the issue, she finds that Nina is in love with Sergay, despite the fact that he was her father, even if not biologically. Eventually, Sergay reveals to Nina that the 7th was actually when he first met and adopted her and she cheers up.
When Schwarz launched an attack on Windbloom, Nina went off to meet up with Nagi who arrived with Artai's Naval and Army forces. When the two met up with Arika, Mashiro, and Sergay who were facing off with Smith and Erstin, Sergay was injured and a handkerchief that Arika made for her benefactor fell out of his coat pocket. This reopened the old wounds in the fragile Nina who again broke down, her renewed hatred and jealousy of Arika's relationship with Sergay unlocking the power of Nagi's Gem. She used her new robe to try and kill Arika but Erstin's SLAVE took the shot, resulting in her death. Regardless of Erstin's last wish for them to stop fighting, Arika and Nina fight anyway, a battle which ends in a massive explosion.In the aftermath, Nina tries to keep up a cheerful face but in the inside she is hurt and devastated. Nevertheless, she stays by her father's side and remains as Nagi's Otome. Her feelings regarding Arika are still shaky as noted by Nagi but during a testing of the Harmonium in which it is revealed that Nina is the protector and player, she finds Sergay badly wounded. Previously Sergay and a team of Valkyeries consisting of Tomoe and Chie were on a mission to capture Mashiro, Sergay was badly injured by Miyu, who interfered with the battle. Again, Nina blamed this on Arika and viciously attacked with the Harmonium which Arika, Mashiro, and Miyu barely escaped from.
In episode 23 it is revealed that the reason she can wield the power of the Harmonium by herself is because she is both an Otome and, according to Nagi, has Windbloom blood. This and the cradle seen in a picture at the orphanage adds much credibility to the claim that she is actually the real princess of Windbloom. This is also supported as Miyu stated that she found the Blue Sky Sapphire in Artai, where the cradle was found, and took it to Rena's mother and Arika herself, as it belonged to her mother originally. This, added to the fact that Sergay's witness stated that Rena placed the Sapphire Gem on the real princess before going to fight and ultimately dying, seems to indicate that Nina is indeed of royal blood. (Nina also bears a strong resemblance to Sifr, the main character of My-Otome 0 ~S.ifr~, who is the future Queen of Windbloom; Sifr is also voiced by Ami Koshimizu)In the aftermath with the Windbloom forces defeating Nagi, Nina took Sergay with her and watched over him as he remained in a coma in a cabin out in the woods. After an unknown amount of time, Sergay wakes up with no memory, Nina introduces herself and notions how they can start all over again. Nina can now have the relationship she's always wanted with Sergay, not as his daughter, but as a mature woman.
In the anime, Nina carries the Ultimate Black Diamond (漆黒の金剛石, Shikkoku no Kongōseki?), contracted to Nagi, which is the opposite of (or possibly a corrupted form of) Fumi's Pure White Diamond. Her Meister Robe is green and her Element is a pair of sai-like daggers that can be merged into a massive sword. Similar to the behavior of Arika Yumemiya's Blue Sky Sapphire, the Gem reacts to certain situations, specifically Nina's anger and sadness. The Gem was first unlocked when Nina snapped when she found out that Sergay was secretly sponsoring Arika and led to believe that he was having an affair with her. The diamond also seems to work in correspondence with the Harmonium.
The Robe's final formDuring the final battle against Arika, the Ultimate Black Diamond connected with the Harmonium as the Master. This radically changes Nina's Robe (to version 2.0, according to the official site's classification) and she has black wings growing out from her back. She wields Miroku, the black sword with glowing red eyes, similar to Mikoto Minagi's sword from My-HiME. File:Nina black diamond v2
Nina in the manga's Meister RobeIn the manga, it was not Arika but Nina who first tapped on the Blue Sky
Sapphire's power, using its Meister Robe Ver 1.0 (to quote the official website's "rating" ) against Lumen. However, this is only temporary and subsequently, she returns to using the standard Coral Robe.
Eventually, she gains a Meister GEM of her own. This is the Black Smoke Chrysoberyl (黒焔の金緑石, Kokuen no Kinryokuseki?). It is mainly black and gold and its Element is a pair of swords that can be joined end-to-end to form a polearm. Her first master was Nagi. However, when he died to save Mashiro, he left her with his words 'Protect Mashiro-chan'. Then, Mashiro formed a contract with her.
In My-Otome Zwei, Nina gains the Neptune Emerald GEM (海神の翠玉, Watatsumi no Suigyoku?), her element is a Bident(A trident with two prongs instead of the usual three; in mythology, the god Neptune carried a Trident, while his brother Pluto, god of the underworld carried a Bident) and she has a special attack known as the Dash Cold Water Spout.