Method to Acquire - Durandal, Armads, Malte, Miurgre, Aureola, Forblaze, Apocalypse, and MAYBE Holy Maiden have to be intact to get to Chapter 23. Visit the only village on the map with Fir or Bartre.
Damage - He has 20 strength to start, which is VERY good for a Swordmaster to have. He'll deal some pretty good damage.
Critical - He has a pretty good natural critical and Swordmasters have a 30% critical increase. For an even higher critical, support him with Fir and give him a Killing Edge.
Speed - He starts with 23, so he'll hit some enemies twice and others only once.
Dodge - His dodge is in the 60's to start, which is VERY good. He's nearly untouchable in any forest.
Growth Rates
HP - 240%
Strength - 130%
Skill - 140%
Speed - 140%
Defense - 110%
Resistence - 100%
Luck - 120%
Average Level 20 Swordmaster Karel
HP - 46
Strength - 21
Skill - 29
Speed - 24
Defense - 16
Resistence - 14
Luck - 19
Total - 169
Supports
Fir - 10/20/27 turns
Bartre - 20/30/40 turns
Rutger - 59/60/80 turns
Zeiss - 59/60/80 turns
Noah - 59/60/80 turns
Rating - 9/10. He's the second best prepromoted unit in the game and he has the best stat growth rates in all of Fire Emblem history.