Hey there, I'm a strong Fenris enthusiast, and the way he's been done by SmileGate is just sad. So here is a buff idea that actually uses his passive to support his allies. Tell me what you think-- or don't.

Magic Arrow
Instead of granting increased speed to the caster, triggers a Duel Attack from a random ally Hero. (Maybe even a decreased Defense debuff?) 

Unwavering Will
Starts the battle with 50 Fighting Spirit. If an ally or this hero is attacked with a counterattack, extra attack, or Duel Attack, gains 50 Fighting Spirit and triggers Soaring Arrow. 

Soaring Arrow: Leaps backwards and attacks all enemies. Instead of dispelling one debuff, he will gain increased Attack (Greater) or increased Attack for 3 turns depending on Awakening status. Increases Combat Readiness of the caster by 90%.

Strike of Fate
Attacks the enemy with resolute arrows. If the caster is granted increased Attack (Greater) or increased Attack, dispels all debuffs from all allies. If the caster has no buffs or any debuffs, instead dispels two debuffs from all allies. 
If Harsetti is on the enemy team, Strike of Fate is available at the start of the match. (Only applies to Arena)

How about it? Any tips or suggestions, please feel free to comment. And SmileGate...save my boy Fenris