You absolutely need to build up Grave, since there's a stage a couple chapters in where he has a solo fight where you can get stuck if you save over your file during the intermission.
Other characters get forced into the party for some battles, but most of the other instances are easy to work around since there isn't a failure condition if they die. Though I think Okita, Yuri, Grave, Hiro, Carmaine, and Zeonsilt get forced into battles in the last chapter so it might be worth using them a little bit at least.
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The only certain death in the universe is Mother Hitton's Littul Kittons.
Not changing this line until I beat Seven Heroes (2/24/07)