Cynthia appears in a house in Undella Town, where she can be battled. She leaves after being defeated and afterwards reappears during spring and summer. During the springtime, she can be rebattled once a day. After defeating her, Shauntal and Caitlin can be seen visiting this house in the spring, and Lenora, Elesa, Iris, and Skyla visit in the summer. She speaks about a young Trainer who defeated her, and mentions that that Trainer defeated Giratina, referring to the events of Pokémon Platinum.
I love Weird things ¯(°_o)/¯