Viable? or Wo dao Fodder?

meh. fodder it

Well, -ATK is not great for poor Karel. He'll have 39 attack at level 40 with his Wo Dao+ equipped. 38 speed, though…

If you're still interested in using him, you can get him up to 45 attack and 43 speed with Life and Death 3 and the +1 Attack seal. Get him ATK Honed, and he'll have 49 - not bad considering he'll almost certainly double. You could try a typical glass-cannon/desperation build…

Wo Dao+
Reciprocal Aid
Moonbow
LaD 3
Desperation 3
Threaten Def 3

When I first got into the game, Karel was my red unit for a couple of months. Then the first TT happened and I was able to replace him with "Not Marth," and since then, he's moved further into the back of my hero box. That's the unfortunate state of Karel: he's okay, but too many reds are simply better than him.

That said, you should have a clear Wo Dao target before handing it over. Don't give it to Olivia (she's way better with the Ruby Sword). Draug is a popular target, as ranged units ensure their own demise if they can't OHKO him with it.