She has a scar, just below her knee. The first time he notices it is when they go swimming during the summer before their junior year of high school, right after they lose regionals. It's about an inch and a half long, and quite narrow. She tells him about how she attained it when she fell of the stage performing, slightly too enthusiastically, a tap number when she was eight years old.
( "It was a learning experience and, in the long run, I believe it has contributed to help me perfect my performing," she says. )
It's this tiny imperfection that just makes her that much more perfect in his eyes. He thinks it might be one of his favourite little things about her.
He like to run his hand over this scar, feeling the slightly raised skin underneath his fingertips. It reminds him how fragile she can be. It just always seems as if she's invincible; she's such a strong person. But, just like anyone else, himself included, she has moments of vunerablity. Moments of doubt. Moments of heartbreak. Moments of completely laying her soul bare, simply hoping for the best. He notices that she seems shows this side of herself to him more than anybody else. He likes to think that she just loves him that much, as he does, her.
( "I feel like I can tell you anything. You make me feel safe and special and loved," she lets him know softly, firmly snuggled into his chest, his hold around her tightening. )
It's after first noticing this small scar that Finn realizes how many things there are about Rachel, which would be so infinitesimal to anyone else, but that cause him to fall more and more in love with her every day. Like the way she scrunches up her nose when she's thinking really hard, or how she likes to wear socks with things like little pictures of sheep on them when she's feeling sad, because they help cheer her up.
( "That way, whenever a lower my head, I see my socks, and they remind me to stay positive and keep my chin up!" she tells him. )
He's constantly learning more about her all the time, trying to figure out the never-ending puzzle that is Rachel Berry, her giving him the pieces. He doesn't think he'll ever get tired of it. He wants to be the one that knows her best. Really knows her. He thinks he's getting there.
( "Sometimes, I swear that you're the one with a sixth sense in this relationship," she'll tease him. )
She certainly knows him inside and out, and he knows her better than he knows anybody else. He likes it that way. He wouldn't have it any other way.