Backstory - Rhiannon, being the youngest of House Treveylan, was raised with the constant reminder that her life was about duty to the Chantry. Her mother would never let her forget it - it was all about the “family image” with her. She had two choices: become a sister (the “right” choice, according to her mother), or take up martial training and become a Templar. Her father loved his “little girl” and had indulged himself in spoiling his youngest by allowing her to train with her older brothers. She was the only girl, so she felt like she always had to prove herself to them. Oddly enough, she was a natural with a sword and shield. Because of this, making the choice to become a Templar was the obvious one, despite being pushed in the other direction by her mother. She also wanted to make a difference and not become a stuffy old sister sitting around in a chantry. Thus as part of her training she was deployed to the Conclave to help keep the peace. Because of the war, resources were spread thin and they needed the assistance of even trainees who hadn’t yet taken vows, no matter how capable.
Age - I have the head-canon that noble houses didn’t send off their children to the Chantry as young as “commoners.” Cullen left at 13 (I think), but he wasn’t a noble. Sebastian, being a noble from the Free Marches, seems to have gone to the Chantry at an older age. I’m figuring that Rhiannon is in her late 20’s and hadn’t yet taken her vows before the Conclave. She was slated to take them right afterwards.
Personality - Rhiannon is a KISA through and through. She didn’t fight against her duty to become a Templar and saw the Inquisition as a chance to serve the greater good. Even after the Conclave, she had full intention on finishing her training, despite having first-hand knowledge of what lyrium withdrawal looked like in having met Cullen. She accepted “Ser’s” training knowing full-well what sacrifice she was making. But she wanted to make it for the greater good. She has a very professional and serious bearing (like someone else we know), but isn’t afraid to crack a joke when the mood is right in order to help boost morale. Due to her martial training, she often favors the direct approach, even if that means sacrificing the few to save the many. She can also be pretty blunt at times. In-game, she uses a pretty balanced mix of all three dialogue response types. Rhiannon is Andrastian, but I wouldn’t say she is pious. I would put her level of spirituality similar to that of Cullen. Appearance-wise, she is what I would think Snow White would be, if she had turned into a warrior. Beautiful - Noble - Fierce - Strong. She also has a scar right above her right lip (opposite Cullen’s), as well as a scar through her left eyebrow. The scars are nothing special - she is a warrior after all. They happen. Prior to the Conclave, she had seen some action, and she often felt like she had to train harder to prove herself as a female warrior. Her fellow trainees would often remark about how fierce she was in the sparring ring.
Relationship with Cullen (and Blackwall) - After joining the Inquisition, Rhiannon poured her full heart and soul into her newly found duty. She saw Cullen as an excellent military commander and as a role-model of sorts. She knew why Templars had such a bad reputation - she wanted to help prove that not all deserved to be lumped in with the “bad,” and Cullen was a wealth of knowledge on the subject. Their relationship started out purely professional (on her end). She looked up to him almost as a superior (since he was previously a Knight Commander) and a romantic thought never crossed her mind (of course she thought he was attractive, but that’s where she saw it ending and never gave it another thought). She had no idea that he thought she was the most beautiful creature he’d ever seen. But duty comes first, so he kept his feelings to himself, despite what his heart wanted. Enter Blackwall. When she was sent to check up on the lone Grey Warden, she was smitten from the instant he blocked that arrow with his shield. Now here was a noble man. He was very open in his flirtations with her and she liked the direct approach. However, after reaching Skyhold, he started to pull away, claiming he wasn’t good enough for her. Mind games were such a turnoff. Well, that did it - he asked her to break it off, even though she knew he wanted the opposite. She thought he was being a coward - a strong person stands up for what they believe in and goes for what they want! Well, wish granted - goodbye! Ever since that night in her room, they really haven’t said more than two words to each other outside of Inquisition business. In the meantime, Cullen was ecstatic to find out that they broke it off. He was not happy at all seeing her with another man, but he was afraid to say anything to her because, like her, he was all about duty and sacrifice for the greater good. But he couldn’t help it - he was done for the night he found her in the snow and carried her to camp. Gradually, their friendship grew, and Rhiannon started to see Cullen confide in her more. She realized that she had feelings for him after he was brave enough to tell her that he wasn’t taking lyrium anymore. She advised against not taking it, for health reasons, and they had a good argument over it. They were challenging each other, but it was a healthy sort of argument. He appreciated her honesty and said that he’d consider her thoughts. It didn’t take long for the relationship to advance after that. They aren’t very open with their affections in public, simply because they are both so duty-bound. But that doesn’t stop smoldering gazes and caught glances, testing each others restraint (a game of sorts). If anything, that makes things more hot when one of them can no longer stand it and snaps (such as with the desk scene - or in a secluded hallway or dark corner, hee hee). I don’t envision the desk scene as being their first time, btw. In the bedroom, things can get a little rough and heavy, but nothing super hardcore. Sometimes he is more dominant and sometimes she is.
Lyrium and Templar Spec - Rhiannon asked Cullen to start taking lyrium again, with the promise that they would find a better way, together, when the Inquisition was no longer needed. She saw him suffering and didn’t want to risk his health any more. She honestly thought it was the best decision,and even though it hurt to ask that of him, he did seem grateful for it in the long run. Cullen was dead set against Rhiannon’s insistence on finishing her Templar training. However, he saw why she did it and respected her for it. In time, he even supported her decision and helped her acclimate to taking lyrium. Over time, they helped each other keep a reign on the addiction, pulling each other back from the edge of going too far.
Summary - Their story is very angsty and tragic. Tormented warriors that found their equal match within each other. They challenge each other and keep each other grounded, while also being each other’s only source of support and relief. He’s the only person she can let her guard down around, and he feels the same way about her. They fight hard, and love hard. It’s a toxic situation, but they need each other.
Post-Game - I’ve added to my head-canon that she is going to work with Cassandra to rebuild both the Order and the Seekers, and find a way for the the Templars to do their job without the lyrium.