Which of the 4 characters should I select at Creed's Mansion?
Each of the characters at Creed's Mansion (Karna, Tyrin, Randolf and Eric) have their own skills & qualities. It really depends on your preference as to the type of character you'd like to add to your force.
Karna is often considered the best choice, as when promoted to Master Monk she can easily be levelled to 99, making her both a super-strong fighter, and a useful spellcaster. It will take time to build her this high, but she is well worth the work!
Tyrin is a skilled magic user, and when it comes to selecting whether to use the Secret Book on him or on Kazin, I'd say use it on Kazin, because Tyrin's spells as a Wizard are much more handy than Kazins. Tyrin has Blaze, Freeze, Sleep and Bolt, compared with Kazin's Blaze, Muddle, Dispel and Desoul.
Randolf is a hardy Warrior, comparable with the other Warriors in the game, however his attack is often better than Jaha's (Jaha's defence is often better than Randolf's though...).
Eric is much the same as the other Knights in terms of stats, and looking at their growth patters, I'd be inclined to use the Pegasus Wing on Rick as opposed to Eric or chester... I wouldn't call Eric an essential addition by any means, although I know many people like to have a Force full of Knights :)
Since you can go back to Creed's after you have received the Sky Orb, and then recruit the other 3 characters, the choice is only really important with respect to the battles following the visit to Creed's, up until the battle against Cameela ... so take someone of a class that is under-represented in your Force, and then go back for the others and train them up!