Sealen is the boss of Chapter 15 in Hector mode of Fire Emblem: The Blazing Blade. He is a member of Eubans' mercenary band and was sent to take back Laus castle. He seems uneasy with Heath and thinks he is not a true mercenary. The chapter can be completed without killing him, however his fate afterwards is unknown.
Also like many other bosses in the game, his portrait is reused in a different color pallete, in this case with Oleg. His Portrait and Pallete seem to match the Minor Boss Donovan from Fire Emblem: Genealogy of the Holy War.
Normal Modes and Eliwood Hard Mode
|Bow - D||Longbow*|
* Dropped upon defeat if not equipped; Normal Modes only
Hector Hard Mode
|Bow - D||Longbow|
* Dropped upon defeat
Sealen's speed is abnormally low for a Nomad. In fact, generic level 15 Normal-mode Nomads should have speed stats that exceed his own, even on Hector Hard Mode. Further limited by his mediocre constitution, Sealen is incapable of doubling any unit with an attack speed above 3. Sealen's longbow and map position more than make up for this liability, however. He can and will freely move to attack any unit that enters into his range, and can do so with absolutely no fear of enduring any direct attacks. Sealen's tendency to move and attack can backfire, however, as it can be exploited to draw him away from the gradual trickle of cavalier reinforcements that normally spawns near his starting position.
While there isn't much of a reason to engage Sealen in normal mode, the Dragon Shield he carries in Hard Mode is a worthy prize indeed.
—Sealen monologuing after Heath leaves