If you junction less GFs to the same character, they'll level up faster. A portion of the EXP the character gains is divided up between all of that character's juntioned GFs. So, if you have six GFs on one character (and one of them was Eden), then Eden would take forever to level up, even if the others are already at level 100, especially because she requires 1,000EXP to level up, just like the six main characters (and Seifer and Edea, but who's gonna' level them up to 100?)(most other GFs only require 750 or something to level up).