Aster's Dialga is the only Pokémon she obtained in Sinnoh.
As a Wild Pokémon
Dialga and Aster's history goes back to her childhood. When Aster walked off from her parents, she was attacked by a wild Pokémon. Dialga appeared through a portal and protected the girl but fled through the same portal afterwards. Seeing this act made Aster realize she wanted to become a trainer and find the mysterious Pokémon to thank it.
When she first traveled through Hoenn, many people rejected her and wrote her off as crazy when she asked about a large steel like dinosaur who travels through portals. Overtime she no longer mentioned her experience.
Dialga's likeness first appeared in Eterna City as a statue, allowing Aster to reaffirm that it was the Pokémon she saw in her youth.
Aster caught Dialga at The Spear Pillar with Sceptile and Jigglypuff's help.