The Atom is the name of two heroes in the Justice League. This article refers to Professor Ryan Choi. The other 'Atom' is Ray Palmer. Ryan Choi does not appear in the JLU show and is exclusive to the toyline.
"A protege of the former Atom, Ray Palmer, Ryan Choi moved from Hong Kong to Ivy University to find his mentor's bio-belt and take on his heroic identity. Atom's main power is size and mass manipulation, which means he can turn to microscopic size and turn back to a normal person."