It's near the very end of the series when InuYasha and the gang are trying to defeat Naraku and for like 7 episodes Naraku is in that HUGE barrier that's so powerful. Kohaku is with Sesshoumaru the whole time.
Chat with our AI personalities
*Mt Hakurei Episode 111
episode 35
Episode 80: "Sesshomaru and the Abducted Rin"
in episode 9
It wasn't in an episode, it was in the 4th movie, Fire on the Mystic Island.