What to call a person with whom you spend time just because?

What to call a person with whom you spend time not because you like them but because you haven't found anyone better?

EDIT: I am asking more about a romantic kind of relationship than mere companionship, and about a situation when one person kind of uses another.

I'll throw down on this one. Let's say that anyone who is dating is looking for "Mr. Right", or "Ms. Right". But they haven't found them yet. The person they date while waiting may be referred to as "Mr. Right Now" (or "Ms. Right Now").

I might suggest that you are friends with (or dating) that person by default:

a selection made usually automatically or without active consideration due to lack of a viable alternative

I'll also point out that Urban Dictionary includes the formulation "default friend", which may be understood by some listeners: it's grammatically sound, although sounds somewhat unusual to my ear. The formulation "boyfriend/girlfriend by default" or "we are friends by default" is likely to be more widely and readily understood.