Character Analysis
We don't get too much background on Charlie's girlfriend, Susanna, but we know she's a nice woman who struggles with Charlie's selfishness and the fact that, at the beginning of the movie, he's basically a dunce when it comes to emotions and communicating. She doesn't approve of the fact that he kidnaps Raymond, and she finally loses her temper at the way he's taking advantage of Raymond and leaves Charlie to figure his stuff out on his own.
Of course, she comes back into the picture later, when Charlie has cleaned up his act and started just generally treating people (and Raymond in particular) better. One thing about Susanna that is key: she is always nice to Raymond. She even shares a nice (innocent) kiss with him in an elevator in Vegas, while she's giving him an instructional on how to kiss (so he knows for dating purposes).
You'd think he'd freak out about that kind of contact, given that he rejects his brother's hug, but perhaps the difference is that Susanna knows to take things nice and slowly/gently with Raymond.
So, the bottom line? She's the character with the most emotional intelligence to offer, and that's a big help when it comes to Raymond.