if by left behind you mean put in the PC, nothing. whatever Pokemon is first in your party follows you
behind your pokemons
talk to your first party Pokemon behind you.
It is in Viridian City, behind the Poke Center.
The lady behind the counter in the underground tunnel in goldenrod city.
She is in the house behind prof oak's lab in pallet town.
Try to hit the other Pokemon from behind. Try not to be stressed
You can get into that door by hacking it or activating a cheat on it
No, the mechanics behind IVs and EVs have not changed in Pokémon HeartGold and SoulSilver, they have remained the same as they were in Pokémon Platinum.
Behind the Gym is water. Surf on it and enter the cave opening. Then pass the Dragon's Test
it's behind one of the tubes and then you go back and give it to the master dude
If you mean they will be behind you throughout the game, then no, they won't. The only games that allowed Pokemon to follow behind for the adventure were Pokemon HeartGold and SoulSilver. In Pokemon Diamond, Pearl, and Platinum, you can go to the Amity Square, which is connected to Hearthome City, where certain chosen Pokemon can follow you for that area, however.
I'm not sure but my best guess would be get a Pokemon that knows head but and HIT THOSE TREES LIKE THERE'S NO TOMORROW