Cassia Orsellio Romance Guide
Gender lock: male Rogue Trader only. Cassia will not enter a romance with a female Lord Captain.
TL;DR: Recruit Cassia in Chapter 1, then court her on the bridge by treating her as a person rather than as your Navigator. The defining choice is telling her she is beautiful, and the Chapter 2 birds scene is the one beat you cannot skip.
When it opens
entity:cassia Cassia joins during the Chapter 1 questline Secrets of the Navis Nobilite, after the showdown that pulls her away from her home aboard Eurac V. Once she is part of the retinue, the romance becomes available the next time you speak with her on the ship's bridge. This is the slowest and most formal of the courtships, so settle in for a long build rather than a quick fling.
The throughline
Everything about entity:cassia Cassia's arc rewards one attitude: see her as a woman, not a entity:navigator Navigator. The Navis Nobilite raised her as a tool, an instrument of the dynasty, and she expects to be valued only for what she can do. Put her person first over her duties and her mutations every time the choice comes up, and the relationship moves forward. Treat her like a resource and it stalls.
Step by step
Talk to her on the bridge and ask about herself. As the conversation about her life and her place aboard the ship develops, choose the line "You are beautiful, Cassia. Do not let anyone or anything convince you otherwise." This is the cornerstone of the whole romance. entity:cassia Cassia has spent her life being told her mutations mark her as something other than a person, and this answer is the one that lands.
Travel between systems and let the automatic bridge scenes play out. These short conversations fire as you move from one star to the next, and your job in each is to be supportive of her. Back her decisions, reassure her about her standing, and stay patient. She does not move fast.
In Chapter 2 a dialogue option appears to send birds to her room. Choose it. This is the single beat you cannot miss if you want the romance to continue. Skip it and the bond stops deepening no matter what you said earlier.
What breaks it
entity:cassia Cassia rejects intimacy attempts in the early game. Push for physical closeness before she is ready and you set the relationship back. Repeatedly forcing intimate options, or joking twice about ending the relationship, triggers a breakup. The lesson is restraint. This is a romance of slow trust, and you earn it by waiting for her rather than pressing.
Exclusivity
You can court more than one companion at once in the early and middle game. That ends at the quest The Lord Inquisitor and The Lord Captain, where the game forces you to pick a single love interest. If entity:cassia Cassia is the one you want to keep, make sure her romance is in good standing before you reach that point, because every other thread closes the moment you choose.
Where to take her in combat
entity:cassia Cassia is one of the strongest support characters in the game, and her Navigator powers reshape fights the way an entity:officer Officer's commands do. If she is going to ride beside you, build her to do real work. See the Cassia Navigator build for her entity:grand-strategist Grand Strategist setup, the companion tier list for where she lands against the rest of the roster, and the romance hub for the full list of who you can pursue.