Radija can be encountered outside the ornate door that leads to the rooftop. If Kheteric Thorm is already up there, the player can ask her for an audience with him, which she invariably refuses.
She provides a somewhat early hint that the cult of the Absolute goes well beyond mind flayer infections, as her dialogue will reveal that despite her seemingly high status within its ranks, she bears no tadpole of her own and even looks down on the player for being a "True Soul". This mentality is somewhat reflected among the other high-ranking Myrkulite necromancers serving General Thorm, with Kressa Bonedaughter bemoaning the weak servility of the infected she's presented with, and Balthazar using the player's worm-infested status as an expletive.
Radija is a Wizard, and will be joined by her Necromites. Generally, she will cast Fireball and then use her Bonus action to create another necromite with Incubate Death.