Tharja deeply regretted joining this battle. They were fighting for a pointless cause, and she did not feel comfortable killing anyone for Gangrel. She hated him and what he stood for, but she was at a loss at how to proceed. She watched as the Plegian forces clashed with the Ylissians, her eyes coming to rest on a man wielding an huge ax.
She watched him with interest. He was unnaturally handsome, and very feminine for a man. The white garb of a monk adorned his frame, and his beautiful golden hair flowed around him with every swing of his ax. She caught a glimpse of his face, which was twisted into an anguished look, probably due to the demise of his comrades. His golden eyes were like fire, and Tharja's eyes widened at the darkness that suddenly radiated from him.
"How interesting..." Tharja wondered aloud, as she watched the monk strike down another of his foes. He was quickly becoming surrounded, and would soon be overwhelmed unless aid arrived. She noticed that the Ylissians were not going to get to him in time, so Tharja tightened her grip on her tome and ran to the monk's aid. She was very interested in this darkness inside him, and she could not let him die before she had a chance to study it.
The monk was engaged with two foes at once, so he did notice an enemy mage approaching from behind. Tharja quickly placed herself between the monk and the fire spell that was launched at him. She grimaced as the spell hit her shoulder, burning off some of her clothing and leaving a slight burn. She quickly countered with a Nosferatu spell, easily killing the weaker mage and absorbing some of his life force to heal herself a bit. She then felt the familiar sensation of a healing spell being used on her wounded shoulder as she quickly turned around.
The monk was standing beside her with one hand gripping his ax. The other was holding a healing staff, which he was using to cast mend on her shoulder. He looked up when she turned around and smiled an impossibly beautiful smile at her, causing Tharja's cheeks to heat up slightly.
"There. You should be alright now." the monk said as he lowered the staff. He paused as he turned around to intercept an attack from an enemy, and the monk cut him down with ease. Tharja was impressed with his fighting skill.
"I must say, I did not expect to be rescued by a Plegian. But you have my thanks regardless." he continued, the smile not leaving his face. She could see in his eyes, however, a glint of suspicion. This did not surprise Tharja; she would have been wary too if she were in his position.
"You fight well, monk. I couldn't just let you die here; you intrigue me too much." Tharja said with a shrug. "I hold no love for Gangrel and his useless war. Such a pointless waste of life." She muttered the last sentence as she gazed around at the bodies that littered the battlefield.
"May I ask your name?" the monk said, his golden eyes meeting her purple ones.
"Tharja. What about you, monk?" she said.
"I am called Libra." he said. He smiled at her again and placed a hand on her newly healed shoulder. "You have a good heart, Tharja."
Tharja narrowed her eyes at him and shrugged his hand from her shoulder. "You know nothing about me, monk." she said with a harsh edge to her voice.
Libra chuckled in response as he let his arm fall to his side. "Ah yes, how rude of me to make such an assumption. Well, since you've seemed to defect from Gangrel's side, will you join our cause?" he asked, tilting his head at her curiously.
Tharja thought about it for a moment. It wasn't a terrible idea. Having powerful allies was always a good thing, and defeating Gangrel was a good enough reason to join the Ylissians. It would also give her the opportunity to study the darkness that she still sensed deep inside of Libra's heart.
"Hmm, alright Libra, I will join you. For now." Tharja said, nodding slowly. Libra smiled and hefted his ax over his shoulders.
"I'm glad to hear that, Tharja." he said. He looked in the direction of the main Ylissian force and a serious look crossed his face. "Something is happening, let's go." Tharja nodded and they both began running towards the commotion, as Exalt Emmeryn's voice echoed over the battlefield.
I really like this pairing (if I completely ignore Tharja's last line of their S support), and I tweaked the story a bit to have Libra be the one to convince Tharja to switch sides instead of Chrom. I also like the idea of her being drawn to Libra's darkness instead of Robin's, so I'm going with that.