Human Child of Hidden Truths : 
Sereinae / 瀬霊苗
Aka 
Sereinae Replica / 瀬霊苗・レプリカ

General Information :   
When under Okina’s control, her full name becomes Sereinae Matara / 瀬霊苗・摩多羅
Name origin : Spiritual (霊) current (瀬) to a seedling (苗), 霊 comes from Reimu and 苗 from Sanae.
Age / Size : About 20 (physically, but was created a few years ago) / 162cm
Species : Human, very mostly.
Gender : Female
Eyesight : Pretty bad
Physical abilities : Can stand, walk, or run, but can’t float/fly without external help

Abilities : 
Drawing out memories (of a place, for example) from others, turning those into reality.
Imitating the combat style of the people she's been crafted from through dreams.

Details about the Abilities :
This is an extension of Okina’s and, by extension, Satono’s mental energy manipulation ability. By drawing out memories of someone, she can make a mirage of a place like that someone remembers it in a pre-made blank space inside of a specific door (i.e the Phantasm stage doors). She can use whatever power that is tied to that place, hence why she could use season-related spell cards for example. Her book that she holds is the tool that is used for that ability. A book contains sentences and words that recall memories after all.
Since she dreams of the experiences of the people she’s been crafted from, she’s then a perfect imitator of their fighting style. She can then make new attacks based on their abilities. Still, no matter how good she may be at it, the originals will always be better and more faithful than her imitations. She can imitate only one character and nearly only once in a fight. However this ability has been pushed to the extreme in her form where she’s possessed by Okina’s powers where she imitates many characters in that fight.

Profile:
A servant crafted by Okina herself. Her existence is due to the merging of parts of the life and mental forces of many people, Okina being the exception. She has been working for Okina in front of the scenes, serving her while undercover, while the other servants do their work behind the scenes. Since she has no “former life” of her own, Okina wouldn’t feel remorse for stripping that “life” away.
In her spare time, she either helps the fellow servants in their duties, or, more currently, befriends the persons she’s been crafted from, despite the fact that she already knows them makes her uneasy, for not knowing her in return, or the fact that the too curious people might find her too awkward or creepy, or other people might just like her for being nice to people. 
The circumstances of her creation caused her to see in her dreams visions of those people that allows her to understand their feelings and experiences of their lives, making her become their second conscience, due to being a sentient being herself and the only one being able to fully understand them. 
Any good or bad consequences on these people from her would be felt back at her, meaning that she wouldn’t be willing to betray or do anything against any of these people, making her one of the most trustworthy persons to those who open themselves to her. If she’s aware that they’re having ill will, she would rather question them first and act by force against them if nothing works. Still since Okina remained the exception, it would make Sereinae become rather gullible towards her, despite the pseudo-parental affection Okina has towards her.
She would do her best to reassure any of these people in case something is wrong, since she would feel it and understand it easily, and use the point of view of different people to show them different perspectives of thought. 
She’s sometimes wanting to act as the big or little sister, something Okina found as an altruistic act from her, but the fact that these people don’t know her to begin with makes things difficult, making her a socially awkward person. Her general personality is one of a calm and cheerful person, though rarely, her identity crisis due to her creation can make her personality shift, causing her to be bird-brained, mischievous, prideful, rebellious or crazed. She can also on the contrary, shift to becoming a shy or introverted person, and at worst times, she would question where she’d belong the best. 
Spoilers for the events of the mod :
In the latest major events, she has been working for Okina during the four seasons incident, welcoming the new successors to their new duties. She believed that Okina just wanted to get successors for Mai and Satono, but when she realized that first trying to recruit a fairy would be dumb, that this was just a call for attention, and that she was involved within the incident, she fought back against Okina. But regarding Sereinae, this whole event was just a test to check if she can tame Okina’s powers in order to confirm that she is a truly faithful servant. If she doesn’t succeed, that power would overwhelm her and she would be nothing more than Okina’s puppet, quite literally.
The next event was another order from Okina, testing her powers and giving support to a dream dweller known as Sumireko, but she wasn’t aware that she had been involved with a body property dispute, therefore she left during their final meeting to ask for explanations from Okina.
During the beast invasion incident, she went ahead of the protagonists while the beasts were possessing her. Okina’s curiosity makes her want to let her make the crucial choice of who to fight between Keiki and the beasts spirits, and after an argument in her mind between Okina and the beast, made them let loose of their possession, leaving the choice up to Sereinae, whose rebellious personality shift started acting up, and threw the beast spirit away. She ultimately got on Keiki’s side and fought the possessed protagonists as she believes that the beasts are the true culprits, not only daring to use Reimu and Marisa, it was later revealed that their promise to call the incident off was fake, since Saki acted afterwards.
In the past, before the four seasons incident, she happened to have been lurking in some places, at the Dream Palace Mausoleum, at the foot of Youkai Mountain and above the Scarlet Devil Mansion. She took down Futo and Seiran. Her feelings of being only a creation made from other people filled her with frustration, unleashing it on the protagonists, but when she calmed down, she then served as a guide for them, that is in case they won. She ultimately disappears just before they would fight their final opponents. 
In the latter location, she tried to capture Seija Kijin with “impossible” spell cards, but since it’s about Seija, she actually didn’t hold back this time. 

Relationships :
Okina Matara : Her creator. Treats her with parental affection, but also, teaches her to learn how to master the power on the back given to her. Power being the only thing Sereinae can use to exert Okina’s and her own power.
Mai and Satono : Her colleagues. They treat themselves like sisters and share the menial tasks, but Okina orders her look after them. If they happen to do their jobs badly, Sereinae is the one getting punished first. 
Reimu Hakurei : Though she doesn’t know her that well and didn’t ask for anything, Reimu appreciates the help she gets from her in her daily tasks from time to time. Probably as something that she owes her for existing, though it was unintended.
Aunn Komainu : As they’ve both been brought to life by the same person, they feel like sisters to each other, and they each help Reimu in their own ways.
Marisa Kirisame : Marisa doesn’t appreciate at first that Sereinae is part of her. They don’t get along that much and Marisa finds her creepy. Sereinae in return distances herself from her, but still wishes they could get along, and feels sorry for her for having herself exist. But there would be no need to go that far now, would it?
Seija Kijin : When they meet, they disrespect each other and always bully themselves, enjoying it a lot, whether they’re the victim or the bully. They happen to be rebellious a lot. Well, actually, that may be the only real friend Seija might ever get.
The other people she has been crafted from : It’s the same thing, they don’t know her, she knows them through her dreams, but ultimately respects them like members of her family (like siblings or parents). 
The list consists of : Reimu, Marisa, Sanae, Reisen, Star, Utsuho, Tenshi, Patchouli, Alice, Kogasa, Kaguya, Yuyuko, Seija. Note that she’s half their species as long as she imitates them and her personality tends to shift into theirs, at times.
Aya Shameimaru : At first, she seemed to dislike Aya a lot, but only fought against her because of being part of the people of hindrances. Her interest about the tengu made her mind change, and asked Okina’s opinion about what she saw from Aya.
Cirno : When they first met, Sereinae first worried about her overflowing power of her back, but on second thought wondered if the fairies were worth becoming successors as Cirno barged in with her newfound power. She saw her actual potential in one last fight, and hired her, only to be declined.

Design traits :
She has a headband, with a ribbon attached in the middle. The headband, the eye color and hair color are a nod to Fumika Sagisawa (Idolm@sters). The hair is a nod to Reimu, Star and Kaguya as well. The ribbon is a nod to Star Sapphire, to Reimu (contrast from the red color and on which side of the head it is) and Yuyuko Saigyouji (ribbon color).
The glasses are a way to get around a handicap, one being her bad eyesight, so it’s not that different from a wheelchair in that sense.
The jacket has a hoodie that eventually has bunny ears on it, as well as looking like a business outfit. It has a zipper at the lower half. 
The whole business outfit appearance along with the skirt is a nod to Reisen, along with the scarf that looks like a tie, like Reimu’s. Its green color refers to the summer season, Sanae’s hair color, and Rin Shibuya (idolm@sters, yes, again, along with the long brown hair). The pocket’s color is a nod to the yellow color of the stars and the fall season. The blue color of the jacket is a nod to the winter season.
Underneath her jacket is a light blue shirt with white elbow-long sleeves.
Her purple skirt is actually a sleeveless dress, with diamond-shaped patterns at the bottom and at the end of her sleeves, representing her loose links with other people. The purple color is a nod to the spring season and Kaguya’s outfit (even if hers is closer to pink), and is of the opposite color of Okina’s skirt.
When working actively for Okina, she wears an apron like douji servants wear. She wears it while cooking or doing house work as well.
Her mostly blue colored design and blue eyes completes the combination of the colors of the four seasons associated with the eye colors of Okina (fall), Mai (summer) and Satono (spring). Sereinae’s blue eye color being winter.
The white boots are a contrast to Okina’s black boots. Unlike hers, which are heeled black boots, it’s easier to walk and run with those. Its white color is a nod to the white snow and the fifth season.
Her “wings” are actually a butterfly-shaped door attached to her back. She can detach them anytime she wants. They act as gliders as a way to work around her non ability to float.