He knew Mara was about to share something big, something serious, but he had all of five seconds to prepare for the info dump. Phil thanked all sorts of higher powers for the fact that he could continue to keep his expression open, inviting and encouraging without changing all that much at all as Mara spoke. He wasn't nearly as familiar with the story of Luke Skywalker as he would've liked to be, but he understood the gist of it enough to piece together where she might've fit into the story he knew. Phil let her talk, giving her hand a squeeze every once in a while to let her know that he was listening.
"I know a little," Phil replied honestly, putting his arm around her shoulder to draw her in closer. Mara clearly needed some kind of comfort, but he wasn't going to push her for it too hard if she pulled away from him. Hearing more of her story explained to him why he was so drawn to her. She'd also be better off talking to Natasha, honestly, but Phil could prime the young assassin for the older one to mentor.
"I was once sent to kill a young woman in a situation very similar to yours. She had a list of confirmed kills a mile long, an even longer list of kills we couldn't definitively tie back to her, and she was younger than you are now." It had been a long time ago that Clint made a different call, bringing Natasha In and breaking orders to do so. Phil had backed his play and when he'd come face to face with the redheaded assassin, he'd fought like hell to make her part of the organization.
"I'm the guy who plans things. We had a great plan to get her. It was a confirmed kill on sight. She'd taken out every agent we sent in close, so I put my guy on it who could see from miles away. Except he took one look at her and got close, only he didn't kill her." Barton had a way of ignoring orders when it suited him, but he always had good reason for it. "She didn't know any other way to be, didn't trust the agent I sent after her, or me, or my boss, not for a long time. She didn't know what to do with us, because we treated her differently than anything she'd ever known." Phil gently rubbed her back as he spoke and shared his experience.
"The thing to remember is that here, you don't have to be the same person you were. You might have to deal with people you know or people who know you, but that doesn't have to mean that he'll hate you. Chances are, he feels that connection too, and who knows? Maybe when he looks at you, he sees the same thing I do." He gave her a warm smile. "You're a bright, passionate young woman who's got a lot to offer to anyone with half a brain. And Luke Skywalker? He's got at least half a brain." That part was mostly teasing, in the hopes that he'd get her to at least chuckle.