|As a unit, Karel is most alike Legion, being a fast, hard-hitting glass cannon with surprisingly high HP. Karel in particular actually ties his HP stat with Chrom, a decidedly bulky unit. Though Legion possesses 6 more Atk than Karel, he beats Legion out in everything else except for Spd. This, and his ability to have beneficial IVs, let him fill Legion's niche a little better.
With +Spd and Fury, Karel reaches 41 Spd, making him fast enough to double most units outright. Buffs can help this out a lot, but even on his own his large health pool lets him make do by himself. In fact, because of his above-average health, he's a fantastic candidate for Fury, and the recoil damage doesn't afflict him too much. It also helps his statline rise quite a bit, giving him solid Def and good Atk for a Wo Dao user. Life and Death plays more to his strengths, giving him even greater Atk and Spd, and his large health and Spd let him survive at least one attack even with the hit to his defenses, but it won't feed into his Reprisal and negates his solid Defense, letting him tank one hit and one hit only.
His Special depends on how fast you expect to clear maps with Karel. Moonbow deals more reliable damage than Reprisal, but lacks the consistency to its damage that Karel could take advantage of with Reprisal at minimum health. Regardless, Wo Dao will add 10 extra damage anyway, compensating for his middling Atk quite a bit, so the decision between them isn't that important.
The decision between B passive comes down to what phase you expect Karel to be the most prevalent on. Desperation is great for player phase Karel, since he can double all but dedicated speedy units, and if timed right, can let him unleash a devastating Wo Dao-powered Special. Vantage Karel is great on enemy phase, his sizable health pool letting him tank at least one hit and put himself in Vantage range. With Wo Dao, he can punish enemies harshly for daring to attack him with his special up.
Panic Ploy, while a costly skill to obtain, works wonders on Karel and lets him carve through enemies that rely heavily on buffs. Because he has a large amount of HP for a speedy unit, the list of units Karel can activate it on is actually large. If foes are relying on buffs to attack, stick Karel in front of them and watch as he carves them in two. Time to feast on Horse Emblem! If it simply isn't feasible to obtain a Luke or you aren't willing to kill your Valter, Savage Blow lets him soften up targets he plans to attack later.