Sadeena/Chronology

Volume 4
When Raphtalia found Keel in Idol's dungeon, she proved her identity by remininscing about when Sadeena saved Keel from drowning.

Volume 5
Just before the wave at Cal Mira Island, Raphtalia wishes that Sadeena was there to fight in the wave. She was revealed to be the best swimmer in Lurolona Village and was like a sister to Raphtalia. Naofumi admits that a swimming-type demi-human would be very useful when fighting in the water.