Upgrade Iruka, he's universal. works with all mains in various lineups and he's used fairly often for pvp and pve.
Mood scrolls are costly so it really pays to know who you want it upgraded on because you don't want to spend 2k coupons on ninja with less use long term.
If you're going to be somebody like me who tends to swap mains weekly to switch things up, having a roster of different ninja makes the whole mood concept more annoying.
So try thinking about how versitile a ninja can be with the main you favour and a secondary main you like if you want to switch.
If you intend on skill breaking Sasuke for example upgrade him as well, quite nice with a lightning main but isn't so much stuck with it.
Kimimaru is a good tank pairs well with Lightning and water main but he's also replaceable with ninja like Roshi who isn't as bulky per say but quite useful in a variety of lineups.
As the guy said above only you know can decide what you want at the end of your day, just give it some thought.