You can change your main character later on (twice a day for free) so it doesnt much matter.
However...
Because you will most likely lack ninjas early on, my personal preference is Water main (shark bomb neurotoxin experimental team). Noted that Fire main might be better due to mirror ability and early ninjas are not immune to debuffs. So as a support, fire main is better.
So, I would start as Water main and unlock Fire main as 1st option (around lvl 40 or 50).