Long story short, Taiga is Sayami`s best friend and has been with her since they were little. They trained together, and it showed that taiga was superior to Sayami in every way, shape and form, but since Sayami`s father was in charge of the Reapers then he passed leadership on to his underqualified daughter along with the grand Reaper Weapons. Taiga is still much more powerful than Sayami, rivaling the power of the Old Reapers that met an untimely end to a very powerful demon. Taiga serves Sayami as her right hand and best friend, and will do anything for her.
Taiga is part phantom, and her mother was a magically inclined human so she`s able to use some sealing magic. her father was as skilled in the art of the sword as Sayami`s father, if not more so, so she is able to use the Double Sword style just like her best friend, even taking it a step farther to learn the Multiple Sword Style and the Art of the Invisible Sword.