Sei is a rather well-built woman with black plaited hair and brown eyes. She wears a gray and brown robe with a fox fur collar, gray-brown leather elbow gloves each wrapped with three fasteners, and brown and gray pants with a brown leather belt worn diagonally around her hips. She has red flame markings on either side of her eyes and cheeks, and the brand of the Blackspine Mountains near her shoulder, seen when she moves her collar aside. In her dragon form Sei has a red-brown body with white and grey scales, and has white crystal-like spines running down her body from her forehead.
Sei is stoic, straightforward, and reticent, disliking needless chatter and talks only when spoken to, except in the case of Dom. She is quite solemn, angry most of the time, and doesn't often appreciate jokes at her expense. However she has a good heart and will go to any lengths to protect or seek out those she cares about, using her powerful fire-magic to her benefit. She also has a soft spot for her kin, evidenced by her reunion with her grandfather and her childhood friend Anton.
She calls those who are not affiliated with her village as lowlanders, indicating high regard for her people; in fact, she threatened to kill Dom on the spot when she thought he faked his brand just to fool her. She is quite confident of her abilities and belittles competitors, as when she ferociously rebutted Azura who compared their abilities, and as she replies smugly to Kenna when the latter threatened to kill her should she (Sei) hurt Dom during training.
Sei also has low expectations for Dom, being surprised to find herself less brave than he is when he sat on fire first, or when she refused to believe he could take down an airship in his dragon form. However, despite her pride she gives praise and respect when they are due, when she praised Dom for accomplishing various fire techniques or when she held Leon Stirling in high regard during the latter's wake.
On a lighter note, Sei is an alcoholic. She offered Dom a drink after Anu failed him in his training, and in Book 3 Chapter 3 was elated upon hearing Val offer the group some "Skullcracker Ale" to mourn Leon. She deemed "lowlander" ale to be as potent as rainwater, and bragged how Blackspine's drink, Ember Wine, is superior. Drunk, Sei behaves jovially, even singing and dancing with Annelyse in front of everyone.
Sei's main ability is pyrokinesis, just like those born in the Blackspine Mountains, and uses anger as fuel. She is skilled enough to create massive balls of fire, shape the flames into tendrils, and manipulate heat remotely so as to set others on fire from a distance. She also has enough understanding of her abilities to teach others. According to herself, her flame is more powerful than Dom's and Anton's.
After completing her training with her grandfather in the Blackspine Mountains, Sei has the ability to transform into a massive dragon and back to human voluntarily. This form gives her access to flight, superhuman strength and durability, sharp talons, and the ability to exhale violet fire. It also makes her Kenna's most powerful ally, allowing her to overpower Azura's lightning bolts singlehandedly and leave her severely exhausted.
The only drawback of this ability is that it has a time limit, draining Sei or any other user of energy if used extensively.
Limited Heat Immunity
Sei, like other fire magic users, cannot feel nor be hurt by heat below a certain temperature, and this immunity is dependent upon their endurance and strengthened through training: the higher the temperature, the harder it is to resist. It is implied that, as she is undergoing the same training as Dom, Sei can withstand heat at least the temperature of lava.
The Crown & The Flame
Sei is first introduced as Hex's fire captive, she intends to use Sei's magic to use the cannon against Kenna's army in the Battle for Stormholt.
When she meets Dominic for the first time, she recognizes the brand on his chest and reveals that his mother was a member of the mountain tribes.
In Book 2, Sei offers to train Dom to control his fire powers. Dom travels to the Blackspine Mountains to learn from Anu Rhuka, Sei's grandfather. Sei and Dom successfully turn into dragons to help Kenna fight in the final battle, but Dom is captured by Hex.
In Book 3, Sei takes her dragon form and assists in rescuing Dom at Aurelia.
Anu is Sei's grandfather. He affectionately calls her "little Sei-Sei". Even though Sei has got problems to show it, it's clear that her grandfather is very important to her. She even lost her will to live when she thought that Hex had captured him.
Playing as Dom allows the player the choice to flirt and romance Sei, but she doesn't respond very well to his advances. It is hinted that Sei has romantic feelings for Dom in Chapter 4, if the player chooses to kiss Sei as a distraction, she will feel happy for a split second before hastily telling Dom off at his bad timing (the kiss). Sei is extremely upset when Dom is captured by Hex near the end of Book 2.
Although she is rude to Dominic, she eventually grows fond of him, but she doesn't show it very well. An example is when after Dom fails his training, Sei goes into the forest to encourage him not to give up, which shows that she cares for him in her own way. When Dom was captured by Hex, Sei broke down in tears and became determined to get him back.
If the player chooses to drink with Sei, Dom and Sei will share a moment together, and Sei will reveal that she accidentally killed someone when she was a younger, more powerful fire-user (someone she cared about, it is possibly implied to be her grandmother) because of her ignorance when using her fire magic. She becomes sad and upset and Dom can choose to comfort her by holding her hand or kissing her.
In Book 3, Chapter 14, Dom can choose to spend some romantic time with Sei. She offers him to give some private training in the forest but plays an ancient tradition called Fal Ignis.
Sei and Anton are childhood friends, they grew up together. Sei even tells Kenna not to hurt him while getting him freed from Hex.
Sei and Kenna did not exactly hit off well at the start, but they bonded eventually over their mutual fondness for Domain, and become best friends in the end. By the third book, Sei becomes devoted to serving Kenna, obeying her orders dutifully without protest nor hesitation, something she does not usually do.
Sei has detested Hex ever since she was ambushed by Nevrakis soldiers under her orders. She openly shows her resentment when talking to her, and felt satisfaction after she tackled and knocked out Hex in a fit of rage. When the scientist was eventually captured, Sei glared at her and mockingly declared that Hex would make a fine pet.