He joins the player's army automatically in Chapter 12 along with Midia, Dolph, Tomas, and Macellan.
After the War of Darkness, he returns to Akaneia's service. It is revealed in the sequel that Boah is indirectly responsible for the events of the game as he wanted a King for Akaneia and pressured Nyna to pretend to love Hardin and propose marriage to him. Despite the best efforts of him and Nyna, Hardin finds out which leads to his depression and corruption. Boah stays in the palace even when Hardin becomes corrupted. He eventually is gravely wounded by Hardin for aiding Nyna, but he lives long enough to explain to Marth how Hardin fell under Gharnef's control and says he is sorry for his mistakes and only wants Nyna to be safe.
While not directly stated, it is heavily implied that he died from his wounds after talking to Marth.
Boah has very low base stats, and his his growths are one of the worsts, if not the worst. Use him only if you have lost Lena, Maria, Wendel, and Wrys. His only use is his skill of C in staves which can be used in Warp skip playthroughs. It is not recommended at all to Class Swap him, as his only real skill is healing.