After you beat Regigigas and unlock the concealed ruins go back to Registeel and battle him again. If you don't recruit him try again.
You can recruit Registeel in Pokemon Mystery Dungeon Blue Rescue Team but his recruit rate is 0.01 percent. Registeel can be found at the Buried Relic and his friend area is the Ancient Ruins.
arceus isn't in Pokemon mystery dungeon 2. he is maybe in Pokemon mystery dungeon 3.
no
you cant
they will go back to buried relic
50
yes
aegis cave
who cares pokemon are gay.
I think its charmander and riolo
yes
By defeating foes.