So, I thought I'd share my thoughts for how I go finds out about Piers once he's reunited with Emmet after PLA.
After traveling to Hisui and reuniting with Ingo, saying goodbye to everyone there, and getting Celebi to return them to modern times, Ingo and Emmet return to the hotel in Sinnoh that Emmet has been staying in once he found out how to actually get to the past. They plan on leaving the next day to go home to Unova. Ingo gets to shower first (and he's very happy to remember plumbing and indoor water systems), and Emmet just changes into PJs, removing his gloves in the process.
When Ingo is dressed, he steps out of the bathroom and notices the ring on Emmet's left hand and questions it. Emmet, in his excitement that he's finally reunited with his brother, totally forgot about the crucial detail that he's engaged. The gloves hid the ring and Ingo only noticed it now. Of course, Ingo is supportive but he has a million questions. Who is it? Where are they? How long have they been together? Is it someone Ingo would've known before Hisui? What are they like? When can he meet them?
Emmet frantically tries to answer everything. His name is Piers. He's a gym leader and punk rock singer in Galar. He stayed in Galar to let Emmet have his reunion with his brother and also to stay with Marnie at home. They've been together for almost four years. No, Ingo didn't know Piers personally, but he probably heard about him in the news or from other gym leaders or subway passengers. He's very sweet and a talented singer and trainer. He'll meet him soon.
Little does Emmet know, Piers and Marnie took a trip to Unova to meet them there to surprise them. And Ingo is too busy trying to deal with the fact that not only does he have a brother, but he'll soon have a brother-in-law.
