Okay, so: Mercury's backstory.
Growing up, Mercury was the poor kid. His dad worked in a distant region, sending money when he could. His mom had to work nearly all the time as a clerk in the mall. One day, Mercury's older sister ran away from home. A few weeks pass, and Mercury and his mother get a letter from Mercury's sister saying she's decided to become a professional Pokemon trainer, and send them money. A few years of this go by, Mercury's sister sending home the money she earned in Pokemon battles. Then she got sick with the mystery disease. The doctors said she was a hopeless case. Mercury's sister gave Mercury a Pokeball, then said that she wouldn't be alive to give it to him for his next birthday.
The next thing Mercury knew, he found himself holding a wrecking bar in front of a convienice store with all of it's windows broken in, alarms blaring, in the pouring rain.
So he ran.