You can only get a star when challenging people if only you
don't take out your card even though you lose and continue to
challenge them until you get 5 wins. It doesn't need to be 5
straight wins though. Say if you battle with somebody and you win
and lose some rounds, as long as if you lose and you continue to
challenge them until you collected 5 wins from them, you will get
the star and level up. However if you collect 3 straight wins, the
arcade machine will force you to take out your card and you collect
9000 pts for your 3rd win. Not sure if there is any other way to
collect stars for battling with other player, but this is what I
know.
