* Is called a "Carian weed" by Iji.
* Has typical Carian traits: black hair, white skin, an exceptionally gifted sorceress. 
* Has close ties to the Carians and is well known by Iji and Seluvis. Is also well informed about us becoming Elden Lord and serving Ranni. 
* Holds the Carian Royal House Scroll in her "supposed" portrait.
* I would argue that the portrait isn't even of Sellen, but it is actually of Rellana. 
   * It would make far more sense for Rellana to be holding this scroll and have her own portrait hanging in Raya Lucaria. The Royal House Scroll teaches us Glintblade Phalanx and Carian Slicer, both of which are the novice-equivalent sorceries of attacks that Rellana uses herself.
   * The Carian Sorcery sigil has a blade and a staff. The blade for Rellana and her Glintblade sorceries, and the staff for Rennala and her Moon/general sorceries.
   * It doesn't make sense for Sellen to have had a portrait in Raya Lucaria. While she was gifted, the dialogue from both Thops and Sellen suggests that she was still technically a student, who was likely only a "teacher" to the students of the cult that she started within the school. Also backed up by the locked and hidden room we find in the academy.
   * Why does the portrait look like Sellen (or her crown anyway)? Because Sellen would look like her mother, or perhaps the crown and necklace were originally her mothers. Interestingly, her hair and face under the mask doesn't match that portrait, but actually looks more like Rennalas current hairstyle (ponytail). 

Sellen is the Daughter of Rellana, and Ranni's cousin.