Nikki is one of three possible characters who can help you get into Viper Manor. This is everything you need to know about how to recruit him in Chrono Cross: The Radical Dreamers Edition.
How to Recruit Nikki
Out of your three possible guides, Nikki is perhaps the most complicated to recruit.
When in Termina, head to the left and go down the stairs. You’ll see some pink steps leading up to a green ship, and should enter. Go through the first door ahead of you, talk to the man on the left, and someone will burst in looking for Nikki. They’ll direct you to the Shadow Forest.
Exit Termina and head east to the Shadow Forest. Continue along the path to find a man attempting to sing to a monster. He’ll run off and be attacked by Cassowaries which you’ll then have to fight and defeat.
He’ll jump down into an underwater cave, and you can follow around the path to enter after him. Meet Nikki.
Once you’ve done this, it’s a simple case of talking to him and accepting his offer to join your team and infiltrate Vipor Manor.