Construction Zone

Also, ruthless is not all she is; Hank spends most of the "Unstoppable" arc trading one liners with her, so he knows she's witty, he's seen that she doesn't just use other people's lives but is more than willing to put her own on the line (I think that makes a big difference - I couldn't see him going for someone who is ruthless only in the sense of endangering others), she doesn't just talk the talk, she walks the walk; he's seen her bravery and endurance. And in their last conversation in Unstoppable, she tells him that they both lost people on this mission because she made mistakes, and she needs someone unafraid to argue with her so that doesn't happen again. So he knows she's not an ideologue always convinced that it's her way or nothing, or that she's always right, but someone willing to make a change (to a degree), someone willing to look at herself and admit wrongs, which is rare. (And makes her definitely not the thug he initially took her for.) Working with her in "Unstoppable", he's also seen her ability to speak in various (alien) languages he doesn't know, which I think he finds intriguing now that he doesn't suspect anymore this enabled her to set the prophecy up. (So much for the argument that Brand isn't intelligent enough to interest him, which I've seen in places.)

And lastly, absolutely, I think the fact she finds him attractive and unabashedly enjoys having sex with him is a draw. Especially given his last girlfriend broke up with him when the media accused her of bestiality and was ashamed of him.