I take so long with reference sheets, it's such a hassle... At least I didn't miss anything, right? xD If I did, make up your own markings. There are no markings on the back of the head, unless you want to make some.
Thirteen is basically putty, constantly changing and molding slightly yet staying the same - probably half lava lamp. So do what you please with Thirteen. Draw 'her' as any type of animal, mix 'her' markings around. In moderation, of course - I don't think if you changed EVERYTHING it'd even be considered the same character xD
The one thing that stays the same are the long, floppy striped hyena ears. That's the one signature thing.
Personality: Basically a cartoon character. Thirteen tends to over exaggerate her movements and feelings. She can also take very extreme physical harm without dying (just like any other cartoon character!) She talks with speech bubbles, and has no voice. She's a little bit ditzy and gets herself into weird dares and situations often. She will most often be dressed in 20's swing apparel, or any type of a suit - if not, a band shirt, or something really colorful. 13 will get along with anyone, but really could care less about others feelings - she never talks about anything deep, just makes jokes and off the wall comments. That's the deepest she goes, and if you ever try to bring up something 13 doesn't understand, she will probably stop in her tracks as her brain resets and starts to play swing music instead.