“Alm. Your shock is understandable... It's true that you are Emperor Rudolf's only son. And now you have also become the only remaining heir of the royal family of Rigel.”
—Mycen explaining to Alm about Rudolf in Gaiden.
Mycen (マイセン Maisen, Maisen in the Japanese version and Misen in some of the fan translations) is from Fire Emblem Gaiden.
He is Alm and Celica's adoptive grandfather and a reputable former soldier. Originally a high ranking Zofian general, Mycen discovered that Dozah was killing off the heirs to the throne as they were born. He was framed for the crime and banished, but not before he saved Princess Anteze from Dozah. Around this time, he was approached by King Rudolf of Rigel, who entrusted his son Alpine to him, and explaining that the over-involvement of the gods Mila and Duma were leading the people of Valentia to ruin, and that he intended to become a "destroyer" in order to provoke the rise of Valentia's true heroes, even if it meant having to die at the hands of his own son. Upon realizing that Rudolf was willingly consigning himself to hell for this greater good, Mycen could not refuse him.
Mycen dubbed his adopted children "Alm" and "Celica" and initially raised them both in Ram Village, but later sent Celica away to keep her safe from Dozah. He spent the later days teaching Alm his old techniques in the peace of their village. Originally, the rebellion movement wanted Mycen to join them but he refused, claiming he wants no more part in war. Alm and his friends join in Mycen's place. Mycen then meets up with Alm at Zofia Castle, encouraging him to follow the path he believes in and hints that Celica would soon be joining them in the castle, saying that both of them will free the land from its fate. When she arrives, he encourages her to speak to Alm, but leaves afterwords.
He later appears at Rigel Castle after the death of Rudolf, and explains Rudolf's motives for the war. He then joins Alm's army and encourages him to hurry to Doma's Alter in order to save Celica and end the war. After the war he became Prime Minister of the United Kingdom of Valentia and served as Alm's right hand man, putting all his effort into reconstructing the land.
In Chapter 4, talk to him at Rigel Castle (needed to progress).
Mycen is a slightly above average unit, and he can be used to replace characters who have turned out badly or died. He is the last character recruited in Alm's route, arriving just in time for the final dungeon. His starting stats are okay. He has impressive HP, Strength, and Defense. While his Resistance may seem low, the fact that said statistic does not increase in the game and that only magic units and Cliff can compare with it makes him an excellent mage killer. His Skill is average. His Speed is low, and his Luck is the worst in the game. His growth rates are almost non-existent, so he is unlikely to improve much even if you try to level him. The main reason he is viable is that he is quite strong and durable, so even with his lackluster Speed, he should be able to handle fighting late game melee units.
Because he is a Lance user, he can use Sol, Luna and Astra, three powerful spears. With one equipped, can do some pretty hefty damage. Overall, he is a good unit, and actually can hold his own pretty well in the final dungeon.
*Enemy only, joins unequipped
Serves as the prime minister in the newly established government of Valentia. As King Alm I's excellent right-hand man, he threw all his effort into the kingdom's reconstruction.
“Alm, Celica. Surpass me. For the sake of Valentia...”
—Mycen's death quote in Gaiden