Fran,Herc,Ibaraki who should I invest in?

The most important factor is which you like the most. Aside from that, most would say Herc, since he is the easiest to use out of the three. Also, his bond CE is one of the best in the game so it's good to keep him in your party as much as possible.

Depends on who you need. If you need farmer, Fran. If you need zerk who can kill, Ibaraki. If you need backliner, Herc.