Who should I promote?

Valter is accually a rather nice as a flying bruiser. You can go for both offence and defence on him with the right builds.

Jamke isn't bad, but he really is just a Leon without merges and IV

Valter is probably a great option as he can be used for mobility support for armour and infantry units. His Spd is also decent and he can receive Hone fliers buffs. Jamke has a crazy set up but offers less Spd if I can recall probably. Valter also has panic ploy which is a great skill for debuffing foes with visible stat bonuses. He can be built with a suicide fury set, or he can have a defensive build with Brazen Atk/Def or fortress Def. Brazen Atk/Def is superior as it has synergy with Valter's weapon as the effect of Valter's weapon skill reduces his Hp meaning that he can easily be in range of Brazen Atk/Def.

I had like 60k feathers so I promoted Jamke. I gave him Close Counter from the free 4 star Takumi, and Atk Smoke 3 as an C skill, with Close Def 3 as the Seal. Gave the Slaying Bow a Def refine and gave him Glimmer. He works really well in Tempest Trials and Arena