I'm back for a while ;)
The girl with the red bandana, whose anime counterpart is May, is
Sapphire. Sapphire's pretty wild for a girl, and messy. She doesn't like wearing clothes, but that outfit was an exception, given to her by Ruby. She prefers battles over contests.
The boy with the headband and white hair, whose anime counterpart is Brendan, is
Ruby. Ruby would sound girly, because he is very neat, can sew and loves to makeup his Pokemon for contests. He prefers contests over battles.
These two are at the left and right of the picture.
In the middle you'll see two boys and a girl. The girl with the white hat is
Blue, special to the manga until a similar looking character came out in the Gameboy Advance Fire Red and Leaf Green games. She's shrewd and sassy, and was once a cheat and a thief (she stole her squirtle).
The boy with black hair, whose anime counterpart is Ash, is
Red. Red is pretty smart and treats his Pokemon as his friends.
And the boy with orange hair and black outfit, whose anime counterpart is Gary, is
Green. He's the rival of Red, but unlike his anime counterpart, he doesn't really show off. He also seldom talks unless it's needed because he seems deep in thought usually in the manga (I think). He trains his Pokemon rather hard compared to Red.
Note that Green for the boy and Blue for the girl are names used in Japan. There's a switch in the US.
There's also two more characters not in the picture: Yellow and Wally (Suspected to the be the next Emerald). Only Yellow does not own a Pokedex.
Yellow's a cute girl who has special psychic and healing powers. She hates hurting Pokemon and thus hates to fight Pokemon. This causes her to be lousy at capturing them with Pokeballs.