Major Arzen is the central antagonist of Fullmetal Alchemist: The Prototype.
Personality wise, Arzen shares many traits with both Father Cornello and 1st Lieutenant Yoki. Like Cornello, he uses an incomplete Philosopher's Stone to further his own agenda, in his case, fabricating an immortal body for himself. Though, while in the original story, the townspeople adore Cornello, Major Arzen is universally panned as a "greedy bastard".
His role is more similar to Yoki's, ruling over a poor town with absolute authority, although, unlike Yoki, he is always completely callous and never at all comedic.
Role
Arzen appears only in the Prototype, where he is shown to be in possession of a cubed Philosopher's Stone. He lends this stone to alchemists in an attempt for them to fashion an immortal body for himself. However, this always results in a rebound, killing the alchemist involved.
When both Edward Elric and Alphonse Elric visit the desert town of Krowatol (the setting of this chapter), Arzen becomes aware of their exceptional reputation as alchemists and is quick to employ Edward to use the stone for him. Ed initially refuses until he is presented with the stone, along with the fact that both his brother and Rose Thomas are being used as leverage against him. He begrudgingly agrees, though lists two terms prior to the act: 1. If he complies, they will be set free, and 2. He gets to keep the stone afterwards.
Arzen gleefully chuckles, stating it's a rather cheap condition and the transmutation proceeds. Edward swallows the stone before quickly transmutating a new body for Arzen. The major is stupefied upon realizing that his new body is truly invincible, however, he is quick to betray Edward and orders his subordinates to attack.
Meanwhile, Alphonse and Rose are also attacked, in a scene mirroring chapter 1 of the original story whereas Alphonse's condition and Edward's automail save them both.
However, soon it is Edward who turns smug, stating that he was well aware of Arzen's treacherous nature from the start and mixed a single screw into the transmutation, instantly freezing Arzen's body and morphing it into a solid steel statue, which is said to be forever ageless and invulnerable. It seems that the major had finally gotten his wish in the end.