Maelstrom

Well we already know he's got Distant Counter built into Ragnell, which he should so that's good. He also seems to be able to use Aether like it's a low cooldown special. He'll join the group of vantage abusing 1-2 range counterers, with a stat spread that'll probably be very similar to Takumi's. Solid bulk, high Atk, solid Spd, glass Res.

Idk that I'd replace swordbreaker with vantage but that is definitely a great option.

What would be good for his C slot? Threaten Atk, to help make sure Heavy blade gets him his quick trigger?

Assuming it's based on his PoR stats then his Spd will be low 30s, in which case Threaten Spd may be a good option to help him double things and thus charge Aether faster. It'll be interesting to see whether Threaten Spd or Atk wins out, but whichever one does it'll be in the name of getting quick Aethers.