Ostrava will appear in the Nexus if and only if
you rescue him/progress his questline in 1-1, 1-2, and 1-3. It seems that defeating Penetrator will force him into 1-4 (so if you rescue him in 1-3 but defeat Penetrator before returning to the Nexus, you will miss his third visit there permanently). Meeting him in his body form in 1-4 after rescuing him in the three previous levels will cause him to commit suicide and fully despawn from the game (not counting his BP form). Each time he appears in the Nexus, he will be sitting somewhere between the Archstones for Stonefang Tunnel and Shrine of Storms after each of the three occurrences listed above in the spoiler tags, with different dialogue each time.
Here is his Nexus dialogue after his 1-3 event, for those of you interested:
King Allant led Boletaria with a round table of brave knights.
The royal Twin Fangs, Vallarfax and Biorr,
Alfred, the knight of the tower,
Metas, the knight of the lance,
and the brave tribesman, Long Bow Oolan and his fearsome legions.
But today, Boletaria is an abysmal mess.
Vallarfax was lost and Biorr slipped through the fissure, never to be heard from again.
All the rest, along with Boletaria, have been devoured by the fog, and will soon be prey for the Demons.
The Boletarian knights are no longer.
But perhaps our age will see the rise of new heroes, such as yourself.
"Apologize to the funyarinpa! Goodness, you are such a rude woman."
PSN: FinallyIndigo (Finally-Indigo)