Fylja is an Magic affinity Epic from the Dwarves faction introduced to Raid during the Karato Foxhunter fusion in February 2022. The fusion gives you the opportunity to fuse 4 copies of Fylja and she stands as a good alternative to the main fusion if you do not have the resources to get him.
Fylja has an A1 that can reduce enemy MAX HP making it viable for the Scarab boss when paired with the A2 that drops turn meter. She will also be useful against Dark Fae with her A3 AOE skill that grants a lot of turn meter when hit against multiple enemies.
Flyjas passive always ensures she will spike her own damage increasing her own attack and crit damage.
Overall her kit is very selfish and lacks debuffs but I can see it having its uses especially cycling from her A3 to A2 against waves of enemies.