She's really insufferably confident. If he's being honest with himself, it's one of the reasons he agreed to be her partner in the first place. Partially because yes, she listened to his music, liked it, thought it was cool. But also because she found him. Started the conversation, smiling wide and sure of who she was and what she wanted. Him. He isn't used to being the one wanted, isn't used to being approached with such assuredness, and certainly not used to her complete lack of arrogance.
So he smiles, crooked and exuding his own brand of confidence because they are going to be awesome together, and says, "Yes."
It isn't until later, when they've already started their training that he realizes there's more to her than her confident exterior. She's brash, heated and headstrong, and so stubborn. It's annoying, because she's not always right, but more often than not she is, and it's frustrating because she's grown up in this world of Meisters and Weapons and Death Lords and Kishin-eggs, and he's still stumbling along trying to piece it all together. He gets discouraged, though, and thinks more than once that this might be worse than his family and the future he might/might not have had if he'd stayed and tried to ignore the fact that whenever Wes pissed him off he had a tendency to accidentally stab walls and that puberty would be nothing but a mess of torn sheets and ripped mattresses. He thinks that maybe this just isn't for him and he'll leave once he learns to not sprout blades haphazardly, but then Maka stops him one day after a particularly grueling session.
"Soul," she says, and her voice grates on his already frayed nerves. He scowls at her, and she grins at him, wide and unassuming, that grin she first gave him. "You're doing great!" He wants to tell her that she's full of shit, and he couldn't be worse, but there's no trace of sarcasm or mocking in her voice. "We're going to make you into a Death Scythe in no time!" It's that same confidence again, and he feels something, a little sensation in the pits of his stomach. Excitement, hope. He stays, he learns, and when he stumbles, she's always there, dragging him back to his feet, always pulling forward, forward.
For a while, he thinks that he's got her number. Maka is an open book, and he laughs when Black*Star has trouble understanding Tsubaki. At least, Soul thinks he has, and then the divorce happens and it's like someone threw a bomb at his carefully ordered world, and in the end, with Maka's mother gone and father estranged, he's the one left to pick out the emotional shrapnel, to suture the wounds and hold his Meister together. He does it at first because he's not sure what else to do. She's his Meister, and he can't just leave her. It terrifies him to be needed, to be relied on so heavily, even if Maka will never say so. But when he thinks about it, it's really not all that different from their partnership. She has always relied on him, and he's in just as deep.
So he stays and tries to learn this new Maka whose confidence has dissolved overnight, who pushes them to new extremes in their training and nags him ceaselessly about his grades and about slacking off because she's (they've) got something to prove now more than ever. She grates on his nerves more than ever, but if there's one thing that Soul understands, it's family trouble, so he takes her nagging and ignores the sobbing that comes from her room some nights because he knows she'll deny it, and stays close to her any time her father shows his obnoxious face. He's barely a teenager, but he knows what lashing out looks like, tries not to think of the therapist his parents had sent him to and the phrase "transference."
Instead, he throws himself into their training because that's one area where Maka's confidence remains unwavering, and while he's not going to compromise his slacker style when it comes to homework, he is more than eager to fight. They hunt fiercely, and he can't help but revel in the way she wields him, strong and sure and when they take their first soul, he thinks that it might be the perfect moment. Maka stands, panting slightly, but still grinning as he transforms, and together they stare at their first soul. He slurps it down, and she watches, transfixed by the import of what they've accomplished. Caught up as he is, he still sees a little spark in her smile. It takes her nearly another month and a particularly vicious run in with her father before he hears her crying in her room again.
It's almost a full year before things really equalize again, but this time Soul is positive that he's really got her down. The divorce is nearly finalized, despite Spirit's best efforts, and they've got their 99th soul. And then, Blair. Perhaps, given Maka's history, he doesn't handle the situation as best as he could, but they'd been increasingly desperate and she'd started getting frustrated in a way that didn't bode well for anyone. He can't really bring himself to regret it, though. It had worked (for all the good that it did them), and really, by now, he thought that she would have trusted him more. Still, he can't really blame her either.
He's not her father, though. Never will be, and tells her such. He thinks it sinks in, because she immediately brightens, and once again he can clearly see that confident, bold girl that drew him in. But now there's Blair, and surviving Blair, and another facet of Maka that comes to the forefront, most frequently in the form of the spines of books on a crash course with his skull. He doesn't get it (though the part of him that's getting older begins to) because it isn't his fault, it's Blair's, and he can't help being ninja'd by Miss Pumpkin Tits all the time. He blames his chronic migraines on Spirit (can really blame most things on him), and begins to wonder if he's a masochist because leaving never even enters into his mind. He even might (in the furthest reaches of his teenaged brain) be starting to enjoy the way her face gets red and the book descends because he knows what she looks like when she's angry, and that's not it.
Somewhere along the way, between those book spines and the nights he pretends he doesn't hear her crying over a photo album from her childhood (and she pretends that she doesn't notice the box of tissues and the glass of water that magically appear by her door), and the way her self-assured smile lights her eyes and the room, and how she learns that having a plan is nice, being able to say fuck it and wing it is crucial, he realizes that he'll never really have all of Maka figured out. Just when he thinks he'd gotten all of her, something throws a wrench into his understanding, and he has to start all over again. He likes the fact that he finds her socks stuck to his tee shirts and that he's found the Maka that no longer bats an eye when he's doing their laundry and has to fold her panties. Discovering the Maka cuddles on the couch with him is less a revelation and more like a puzzle piece falling into place.
Somewhere along the way, between hard kisses and sweaty bodies and holding hands for fun and more book spines, Soul finds that he doesn't really care that he'll never really have it all down, that finding new Makas is kind of fun and hilarious and an adventure of its own. And really, at the end of the day, whether she's his Meister or his lover (and really she's just both and there's no difference), whether she's broken or hurt or jealous, she's still Maka. She's still that brave, confident girl who took a chance on him and pulled him ever forward.