Colette is a big naive, especially when it comes to trusting other people, as she often trusts people she shouldn't trust, including those who wish her harm. She takes responsibility on behalf of everyone else, and often puts other people and their happiness before herself and her own happiness. She doesn't want others to worry about her and would do anything to prevent this from happening, including hiding her own troubles, such as being sick, from them.

Despite her willingness to put others before herself, Colette wishes to live life as a normal girl. She later starts to believe that no one is made solely from dying. She adopts this viewpoint due to an influence by a friend. She strives to prevent anyone from suffering the same fate she did.

Colette was born and raised in Iselia, a small community located in the north-western part of Sylvarant. When she was born, there was a Cruxis Crystal, an evolved kind of Exsphere, in her hand, signifying her as the Chosen of Regeneration. Her thought-adoptive father and grandmother, Frank Brunel and Phaidra Brunel respectively, raised her in the village as her family. She was brought up with the knowledge that she would need to die in order to save the world, causing her to gain a truly selfless nature, although she hid this fact from her closest friends. At some point during the early years of her childhood, Colette was taught how to read angelic language. As she got older, she was introduced to the clergy, some of whom she would later encounter during her Journey of World Regeneration. She also met the priests who were meant to accompany her on this journey. She then started her formal education at the Iselia schoolhouse, where she met Lloyd Irving. She and the young Lloyd soon became friends. Sometime later, Genis Sage joined in their circle of friendship.
Journey of Regeneration

On Colette's sixteenth birthday, Cruxis sends down an Oracle. Colette, accompanied by Lloyd and Genis, head for the Martel Temple only to find it under attack by the Renegades, who are mistaken for Desians by the trio. Lloyd, Genis, and Colette engage the Renegades until a mysterious mercenary, Kratos Aurion, arrives to defeat and drive off the remaining Renegades. Afterwards, they enter the Martel Temple so Colette can receive the Oracle. At the highest level of the temple, the angel Remiel arrives to convey the blessings of Cruxis and bestow the Tower of Salvation upon Sylvarant. Because of his angel status, he is initially believed to be Colette's "real father", although Colette does not feel like he is her actual father, a fact she admits later on.

The end of the Day of Prophecy marks the beginning of Colette's Journey of Regeneration, which entails releasing the five seals that are scattered throughout the world. The fifth and final seal is at the Tower of Salvation, where Colette's journey will end. Colette will be sacrificed, unbeknownst to Lloyd and Genis, and the dying world will be regenerated. The night before she leaves on her journey, Colette visits Lloyd at his house and says what she believes will be her final goodbye to him. Lloyd, who had just found out that his mother was murdered by Desians, asks to accompany Colette on her journey. Trying to cause her trusted friend Lloyd the least amount of grief, she lies to him about the time of her departure, in the hope that he will then be able to live in the regenerated world.

Colette leaves the village of Iselia under the protection of Kratos and Raine Sage, her teacher. As she is leaving, she acts, in Genis's words, "overly cheerful" in an attempt to hide the fact that the Journey of Regeneration will end in her death. Eventually, Lloyd and Genis catch up with Colette and, having nowhere else to go after their banishment from Iselia, are permitted to travel with her on the World Regeneration Journey. As the party moves from one Seal to the next, Colette begins to gain more angelic powers, including angel wings, enhanced sight, and improved hearing. However, she also loses the things that make her human, like the ability to taste food, sleep, and even speak. She tries to hide this from the party, especially Lloyd, but when she loses the ability to feel, Lloyd tricks her into revealing the truth about her condition, and he goes on to tell the rest of the party. Eventually, they reach the Tower of Salvation, where Colette is forced to make the final sacrifice: her heart and memory.
Humanity taken

Having lost everything that made her human, Colette is now the perfect vessel for Martel's resurrection, since her mana signature is the closest possible to that of Martel. Remiel also admits that he is not Colette's true father, and that Lloyd and his party, who Remiel refers to as "inferior beings", just started calling him that on their own. Colette reveals to Lloyd, through some non-oral form of communication, that she somehow knew what was going on, and thanks Lloyd for being her friend before sacrificing her heart and memory and becoming unresponsive to the rest of the group. After the group battles Remiel, Kratos arrives and reveals that Cruxis plans to transfer Martel's consciousness into Colette's body, denying that it will cause Colette to die. Before Cruxis is able to put Martel's consciousness into Colette's body, however, she is taken to relative safety by the Renegades. Lloyd and the party are later forced to flee to the parallel world of Tethe'alla, taking the soulless Colette with them in the hopes of curing her.

Colette regains her senses after Lloyd carves a Key Crest into her late birthday necklace, but Pronyma attempts to remove it. She apparently remembers everything that happened while she was unresponsive, as she knows Zelos Wilder's name, and has regained the need for food and the ability to feel, although she still has her angel powers. However, Colette has been hit with the rare angelic disease called Chronic Angelus Crystallus Inofficium, which causes her body to slowly and painfully crystallize. Colette finds this out between her reawakening and her first visit to Ozette, but just like her transformation into an angel, decides to keep it to herself, both out of embarrassment and an unwillingness to make her friends worry.

As the journey to reunite the worlds continues, Colette is kidnapped by the rogue Desian Grand Cardinal Rodyle, who plans to use her Cruxis Crystal for his Mana Cannon. His plans are foiled when he realizes that her Cruxis Crystal is now useless, and instead leaves a hologram of himself where Colette is being held, telling the party that they can have her before ordering his dragons to finish them off. The party rescues her safely, but Rodyle's words that she is a "sinful and useless Chosen" remain on her conscience. She believes that his referring to her as "sinful" is a reference to her failure to perform the world regeneration, which she feels guilty about despite knowing that it would only bring about what Lloyd describes as "a false peace".

Colette's disease is introduced gradually, starting with mild discomfort and leading to her collapsing in pain in Ozette shortly before Rodyle abducts her. After she jumps in the way of one of Forcystus' shots, a part of her arm is exposed, revealing that she has begun to crystallize. The party then searches for a cure, learning about Mithos the Hero and Mithos splitting the world into two with the power of the Eternal Sword in the process. Colette is cured after the party escapes from Welgaia, but this is followed up with the revelation that Kratos is Lloyd's father and that a half-elf friend of theirs is in fact the leader of Cruxis, who wants to use her as a host for his older sister Martel.

When the party goes to Flanoir to find a doctor for Altessa, the player is able to choose a character to whom he will talk to at nighttime, and who will affect the plot later on. If Colette is picked at Flanoir, she and Lloyd talk about what makes Kratos a good father, and Colette gives Lloyd a good luck charm to assuage his worries for the future, which later saves his life. Colette is captured in the Tower of Salvation after Zelos betrays the group and is brought to the Great Seed Chamber, where Mithos and Pronyma proceed to merge Martel into Colette's body. Although Lloyd and his once thought dead friends come to help her, it is too late, as Martel has taken over Colette's body. Martel, however, uses Colette's body to condemn Mithos' actions and urge him to stop what he is doing. And when Zelos arrives to place a Key Crest on Colette's Cruxis Crystal, causing Martel's consciousness to leave her, Colette reveals that Martel subconsciously begged her to stop Mithos and put an end to the suffering he has caused. In a skit after the group leaves, it is revealed that Colette was able to understand Martel's feelings while she was possessed.
Final Battle

After defeating Yggdrasill, the party goes to Heimdall, preparing to battle Kratos in order to release Origin's seal. If Colette is chosen in Flanoir, she will approach Lloyd and ask him to talk with her outside, where they discuss Lloyd's situation with Kratos before agreeing to travel the world together to collect whatever Exspheres remain; otherwise, she will be asleep and another party member with a higher affection level towards Lloyd will speak with him instead. After the battle against Kratos and Origin, the soul of Mithos appears through his Cruxis Crystal and tries to possess Lloyd. Depending on whether or not Colette is spoken to in Flanoir and Heimdall, Colette may or may not take the Cruxis Crystal onto herself, resulting in Mithos possessing her instead and taking her to Derris-Kharlan. Otherwise, she will join Lloyd and her other companions in traveling to Derris-Kharlan, where she will be forced to confront a mirror image of Remiel, who mocks and criticizes her for abandoning her duty of World Regeneration. If Colette is taken by Mithos to Derris-Kharlan, her strong soul eventually overpowers Mithos's and, when Lloyd arrives, manages to push him away before she joins Lloyd in finding the rest of their friends.

At the final door, the entire group transports themselves over to Mithos' throne, where Mithos' body is, empty of his soul. Whoever is in possession of Mithos' Cruxis Crystal at the time, loses the crystal, allowing it to float back to Mithos and reawaken him. During the battle, Colette tries to persuade Mithos into giving up by mentioning Martel, but this fails to convince him.

After Mithos is killed, the group proceeds to reunite the worlds through the Eternal Sword and the Summon Spirits. However, the Great Seed dies during the process. Determined to give everything, Lloyd somehow manages to unleash angel wings and follow the seed. Colette follows him, and together they manage to revive the Great Seed, causing the birth of a new tree and its tree spirit, Martel. After witnessing the World Tree's future form, she tells Lloyd to name the tree. At the end of the story, Colette returns home to Iselia, where all the villagers welcome her, and explains that she is no longer the Chosen and wishes to be known as just "Colette". If Colette is picked in the Flanoir scene, she later travels with Lloyd on his journey to collect the Exspheres and begins dating Lloyd.