If Elodie sends away Julianna (Week 2) she offers to tutor her in Lumen arts. However, if Julianna is imprisoned, she will plead on her behalf.
Selene seems a little more openly ruthless than Julianna. She's also versed in the art of divination, which she can use both to find the location of Elodie's Lumen crystal and to detect Togami's Lumen powers during the invasion adventure.
Julianna and Selene are romantically involved. They can be seen holding hands in the royal gardens. It is possible in one of the endings for the two of them to get married and unofficially adopt Briony, Julianna's niece, as their heir.
If Julianna is Elodie's mentor, Selene's status as a Lumen will remain secret unless Elodie reveals it by Attending Services with a high Sense Magic skill, and then asking Julianna about it. Her status will not be revealed otherwise, not even if Elodie asks for the possibility of sinking the enemy fleet with magic during Week 34.
If her status as a Lumen is revealed and Elodie summons the Kraken, it's Selene who suggests both sealing it or sacrificing a young Lumen as ways of pacifying it.
Belying her calm and delicate appearance, Selene is a very ruthless though well-intentioned person who is not afraid to dirty her hands or suggest cruel courses of actions if it's for the greater good.
If she's Elodie's magic mentor, she will not sugarcoat the worst aspects of Lumens or how terrifying their powers can be, unlike Julianna. Also, while she may commit crimes for Elodie's sake (rendering the murderer comatose and killing the treasury guards), she will not have a shed of regret for her actions (again, unlike Julianna, who will regret killing the guards).
Despite this behavior, Selene is truly acts for the wellbeing of Nova and Elodie, evidenced by her quickly revealing her Lumen status (otherwise zealously hidden) and offering to become a magic tutor if Elodie expels Julianna from the capital.
If Elodie imprisons Julianna, then Selene will try to pressure her into releasing the latter. Refusing to do so is the only way Selene will completely turn her back on Elodie (Faith classes notwithstanding).
Selene is a notoriously powerful Lumen with an affinity for water-based magic. Some examples of her power are her usage of water for divinations, locating the exact position of the Kraken in the Novan seas, and in more extreme cases drowing the treasury guards with water summoned out of nothing and force-feeding the murderer salt water. This is discounting what she can do if aided by other Lumen (see Elodie's page).
Besides her Lumen magic, she is an experienced priestess with mastery over divination, meditation and lore knowledge, which she will teach to Elodie as her tutor. As with other tutors and priestesses, it's hinted that this knowledge goes beyond what she teaches in the game.
- Selene is the name of the greek personification of the moon. Besides the mystical theme, this ties with Selene's powers, as it's believed that the moon has an effect over the ocean tides.
- She seems to be something of a homage to Sailor Neptune, considering her water-based powers, psychic abilities, personality, femme appearance and relationship with Julianna (herself a homage to Sailor Uranus).
- A special conversation with Brin (only seen if romanced) reveals that she used to be in a relationship with Julianna, which the latter broke off for a relationship with a priestess. It's pretty much evidenced that said priestess is Selene.