Seifer watched closely while she handled the flask. He was pretty good at reading people when he wasn't completely insane, he had to be to make it through interrogation training, and as far as he could tell every single one of these emotions was genuine. Of course, demons were supposedly pretty good at faking. He'd yet to run into any demon other than the one on their side, who was an exception to the rule, but he still believed it. What was the point of possession if you couldn't masquerade as your victim until the opportune moment to strike? Whether his training would match up with their skill at deception, Seifer wasn't sure, but he wasn't quite cocky enough to believe that he'd win that particular contest.
It was why he had to be absolutely sure that she didn't try to pull some sly trick with the holy water. According to what Seifer had been learning, some demons could pull off telekinetic feats. It wouldn't be hard for a halfway clever demon to use some telekinesis to block off the end of the flask just before the water hit it. It was a clever trick that could give the appearance of drinking the water unscathed while preventing it from actually happening. If his training at reading people wouldn't be enough to spot a demon, his training to spot trickery certainly would be. He watched as she sipped from the bottle and, when he didn't spot any sign of her trying to avoid the water, let out a breath he hadn't realized he'd been holding. Alright, she's just a girl. Just someone that needs help.
He didn't immediately take the flask back. She seemed to like it, so he decided that for the moment, he'd let her hold onto it. Step one, prove that the girl wasn't an enemy waiting to stab him in the back, was done. That meant it was time to move on to step two. If this were someone else, step two might have been to give them the rundown on the war, Lucifer, demons, and the rest of it. This girl wasn't ready for all that yet. Right now, Seifer's priority was to get her to safety and make sure none of the others took advantage of her current vulnerability. Without realizing it, he'd moved this girl from "stranger" to "charge" in his mind, and he intended to see it through.
While she was captivated by the flask, Seifer decided to introduce himself. He lifted his free hand, the other still holding the top for his flask, and gently patted his own chest as he said, "Seifer." Then he looked at her, trying to convey the question with his expression rather than with words. He was hoping someone had at least given the poor girl a name. If not, well, he would have to take that in stride.