Showdown in Pewter City - S1-E5
Character mistake: When Brock's father takes Ash to Brock's house he tells him that Brock has ten younger brothers and sisters to take care of. But during Ash's second match against Brock you can see that Brock has nine younger brothers and sisters. You can even count them when they are all holding onto Ash to prevent him from continuing with his battle against Brock. (00:09:18)
Answer: He is a good guy but just found himself in a bad situation when he worked for Lysander. He didn't want him to harm Ash and his Pokémon when he was trying to get the special bond between Ash and his Greninja to work. He does leave Lysander and go back to being a good person.