[@The1Rolling1Boy]
Thanks! Since all of you are staying in an apartment, I think my character can be someone who ran away from home because s/he doesn't feel love anymore (or something like that). While walking s/he saw the van, noticed that you forgot to lock it, and decided to sleep in the van for the time being. You can see him/her the following day when you guys decided to go somewhere else. That's IF you forgot to lock the van.

Otherwise I'll think of something else. ^u^