One True God-Eater.
I'll be using JP's current 2.0-2.5 bonus for a few examples here instead of NA's 1.5-2.0 because FUTURE.
He's a weird mixture of "Jammer" and "Rainbow" for sure. Before the Rainbow buff, you had to run primary dark because three jammers only gave you x16. BUT you could use things like Persephone and multiple Fenrir uses and it would work out.
Now he can get x6.25 just for matching five colors and x16 for three jammers, making x100 possible on many many boards. Gainaut and Jormangandr are to Fenrir as Kalis are to Awoken Ra in that sense.
You can still use Persephone on Gainaut boards, too. Heavy dark board with R/G/B/J means you can get x64 or, if you're lucky and get more jammers, x100+ still.
Or you can just pile on jammers and go for obscene multipliers that way. Certainly some enemies will do that for you, like Speedster Hermes in Zeus Mercury or Zhou Zu's waifu. Makes things much easier on you. Bonus points if they leave 4 colors behind still, or if they only convert heal to jammer.
As far as subs go. As long as you have at least one of Jormangandr or Gainaut and the rainbow colors, it seems you can run whatever you want or need. Unlike Awoken Ra or Ra-dora or Ascooby Doobis, Fenrir's raw power is dependent only on jammers. You can still focus dark and run Persephone, or you can take advantage of Blue's utility, Green's damage reductions, Isis/Amaterasu/Ceres/Kirin for clerics.
Now that the second Fenrir is skillmaxed, the ability to burst is rarely an issue. Their jammer skyfall usually gives 3/4 jammers for the next turn, so three Dogs rotating on 5 turn CDs is fine. Even Gainaut has a great 8 turn cd for a "DKali + Jammer" board if you need that rainbow damage badly.
His stats are pretty good. Top of the class ATK, 3000 HP is very acceptable, and 200 RCV is salvageable. I've not had any issues with durability yet.
Basically, as far as normal turns go, think "Awoken Ra" damage until a turn comes where you use many many many jammers.
His critical weakness? Skill delays. Sopdet.
edit: the dream is to hit 9 jammers at once with five colors for x625 multipliers. One day, I'm sure.
addendum: for Awoken Horus folk, the combination of Gainaut and Antares seems pretty powerful
