This article is from the Final-Fantasy and other Square Soft Games FAQ, by firstname.lastname@example.org (Nick Zitzmann) with numerous contributions by others.
Character #2 can join you during any disc. Go to the Shinra Mansion in
Nibleheim. There, you have to open the iron safe on the second floor.
Take the key inside, and go to the bedroom in the east wing of the
mansion, on the second floor. Touch the curving stone wall to reveal a
staircase. Go down it. In the pink caverns, look for a door on the
northern wall. You can now open it, thanks to the key. Inside is a
crypt; examine the purple coffin in the center of the crypt and Vincent
Valentine will pop out. While talking to him:
* When you're asked to pick between two choices, choose the
second (bottom one, "Talk about Sephiroth."). Vincent will climb
back into his coffin, so open it again.
* When you're asked a question by Vincent, choose the second (bottom)
answer ("Who are you?"). Vincent will once again lock himself in his
Now, leave the crypt and try to exit the basement caves. Vincent will
catch up to your party and insist on joining you.