More and more Leia was feeling the compulsion to leave, as unkind as that might seem, to drop information of one’s future and walk away - But he was laughing, and looking down at him, Leia didn't know what to make of it.
His master?
Whilst the knowledge that this man was Vader’s apprentice turned Leia’s expression at once, to something close to disgust, somewhat shock; it immediately shed light further on what he’d already told her - the way he spoke and what seemed inherently sinister about him and his connection to the Force, as little as she might know about it all, it didn’t take much to pick up on that.
He spoke of, and showed her a healed wound, having been stabbed, presumably by Vader, and she could only presume, too the fact he was alive had possibly been accidental.
It certainly wasn’t sympathy she felt - something closer to a sort of pity, even in the short time she’d known him, it was becoming clear this man was merely a tool; he'd even described himself as one, likely twisted to- ..whatever this was - he showed a devotion, but it wasn’t loyalty and he certainly made it clear he wasn’t mourning any loss.
"That I have no idea about-" She said in the tone of someone trying to figure out a problem and for lack of anything else to say. She backed up, not in a fearful gesture, her expression was more wary, prepared. "I’ve certainly never heard of you- before the Rebellion began, or after the destruction of the second Death Star." And Vader’s presence had been throughout. "I wouldn’t call it a certainty."
He might be dead, he might not be, she certainly never saw him in Vader’s presence, and Luke and never mentioned such a thing.