>>43620784A large degree of similar experiences, meeting lots of new people, forging an incredible bond. A relationship could 100% blossom, but it could be difficult for both parties depending on how those two treated one another during those 10 years and how old the then Ralts now Gardevoir is. The Gardevoir might see their partner as a life partner/soulmate maybe even a romantic one or just as a best friend who they'd give anything for.
The Trainer may see the Gardevoir as just a pokemon or their bestie. Maybe a lover if they truly got to know one another on a deeper level than just "It slays my enemies and looks cute."