Character: If you find yourself in this kind of situation, it’s important to think about why you see this person as only temporary. Perhaps it’s because they have character flaws. You may find that they’re spontaneous and reckless, which is fun for you right now, but not the kind of thing you want in your life when you get older. That’s fine, just be sure that you’re honest with yourself and know that you are compromising on character. You should be aware of any possible consequences.
If your relationship is truly un-involved (i.e. with limited romance, limited amount of time spent together, etc.), then the character issues won’t manifest that negatively. If they’re irresponsible, that’s fine, because you don’t really depend on them for anything. If they’re uncaring, that’s fine, because you don’t take too much stock in their opinions at this point.
But as a general rule, anybody who plays an important role in your life should have good character, whether it’s a roommate, business partner, husband, wife, or best friend. The closer a bad character is to you, the more likely random problems will spring up in your life. Even if you’re just seeing someone casually, they’re probably close enough that you should hold them to the same high standards.
Let’s say you’re just casually seeing someone, but you find out they’re irresponsible. When they get into in trouble, like in a drunk-driving accident or in a bar fight, do you really want to be associated with any of that? Or let’s say they’re untrustworthy, they might become quietly attached to you. Do you really want to deal with someone who might lash out passive-aggressively?
So, if it’s truly something “just for now,” and unless it’s super-casual, you should still ensure they have a good character.
Chemistry: Perhaps you’re considering something temporary because you don’t have that much chemistry with this person. This person doesn’t excite you romantically, perhaps, but is exciting physically. This is tricky, because a lot of the gestures of love carry an assumption that each partner feels very highly about the other person. How would you feel like if the person you were sleeping with felt the same way about you? What if they enjoy your company physically, but intellectually find you inferior, and therefore don’t consider you marriage-material? That doesn’t sound like someone you would exactly want to snuggle with.
So, in conclusion, you should still seek good chemistry and character, even if your relationship isn’t long-term. If you still decide to compromise on either dimension, just be aware of what the risks are.