Reunion

Reunion is Chapter 16x of Fire Emblem: Shin Monshō no Nazo ~Hikari to Kage no Eiyū~. This chapter is reached if the player beats Chapter 16 in 18 turns or less on Normal Mode (21 on Hard, 25 on Maniac, 28 on Lunatic). Alternatively, the player can fill the requirements by having My Unit be promoted and have a Level 5 or over.

Recruitment
To recruit Katarina, you have to talk with My Unit three times.