There is a continuity mistake on Morty’s self-made ID: he’s actually 17. Cass and Noah are the only primary/secondary characters that are 18 at the beginning of the series. It’s likely Morty either feels he needs to compensate for what he thinks is a shortcoming (his rival Cass being older than him), or he figured he would slightly future-proof the card so he wouldn’t need to make another one in a few months, whenever his birthday is.

In-joke in panel 5: I used Pigma Micron pens to draw these panels. (That was my standard inking tool at the time.)

Everything about Thirteen, including her name, is based on a character created for Evil Jim. In strip #132 of that series, Jim reveals that he created a catgirl assistant named Genetic Experiment 613. I had second thoughts about introducing such a character at that point in the series, so she was killed off as quickly as she was introduced.