(It's no mystery why he called you "Marissa" during sex.) Shut it down ASAP before you're in too deep feelings-wise, and make the text short, concise, and to the point.
If you're serious about pursuing a friendship, propose a fun, but explicitly non-romantic hang-out.
You get her number because you want to go on date and potentially move things further—NOT because you want to stay up all night texting her or asking how her day is going. Keep this clearly in mind: your #1 goal when texting girls is to set up the date. And, as a side note, you should ALWAYS text her the next day.
Every text message should get you closer to that goal. The first text should jog her memory of who you are, and induce some of the emotions of your interaction.
You don't want to get serious with a guy who's using you to a) get over his ex, b) prove a point to his ex, or c) pretend you're his ex.