Mai appeared in DP168. She was spotted unconscious floating down a river in a boat by Brock. He called out to her trying to warn her of an on coming waterfall. But she did not respond. He jumped in the river to stop the boat then used Happiny to pull them out of the water. Afterwards, Mai finally wakes up and introduces herself. Because Brock saved her, Mai asks him to escort her to a flower field. Ash and friends eventually discovered that Mai was harboring an injured Shaymin. She explains that she was approached by Shaymin because it was being attacked by a trio of thugs who wanted to take Shaymin. Mai eventually gets to the flower field that has Gracideas and Shaymin is about to transform in to its Sky Forme. There it defeats Team Rocket and is reunited with its friends. Mai then bids Shaymin farewell.
Holly (Japanese: ホノカ Honoka) is a minor recurring character in the Pokémon anime. She first appeared in Tag! We're It! as Brock's tag battle partner. At first, Holly did not show any interest in Brock, saying that she had no interest in "younger men," but after battling together with him she formed an attraction to him, impressed by his battling skills. Later, in Glory Blaze, she seemed jealous that he was flirting with Nurse Joy. Later on, she was seen to have "heart-eyes" while watching Brock battle. After Holly and Brock lost the battle against Ash and Paul, she couldn't help but feel responsible for the loss. Afterwards, she left without saying goodbye to him. Nurse Joy told Brock that she wanted to train more, so that she could become a better partner for Brock. Bulbapedia
Buck made his anime appearance in DP181 when Ash and his friends were on the ferry bound for Suzuran Island. Unlike the other passengers who were headed for the Sinnoh League, Buck was actually traveling to a nearby island looking for treasures alongside his partner Baltoy. Buck was a treasure hunter, who had found many rarities in the past thanks to Baltoy's senses. With the help of Ash and the others, Buck was able to find the island's treasure and also managed to capture a powerful Claydol. [credit to bulbapedia]
Mira appeared in Sandshrew's Locker! as a character of the day. Wanting to use Dawn's Water Pokémon for her own purposes, she offers to teleport everyone to Hearthome using her Abra. However, she teleports them to a flooded city and tells them her grandmother's pendant was lost in the newly-flooded lake. Everyone goes diving to recover the pendant, but they are repelled by a wild Gyarados. After another confession, Mira explains that actually the Poké Ball containing a Sandshrew she and her friends used to play with was left at the school when the town was flooded because of the dam. Ash had Pikachu defeat the wild Gyarados, and Mira was reunited with her Sandshrew. Filled with gratitude, she had her Abra teleport everyone to Hearthome City.
The Tower Tycoon Palmer (Japanese: タワータイクーン クãƒãƒ„ã‚° Tower Tycoon Kurotsugu) is the leader of the Battle Tower in Pokémon Diamond and Pearl. He is Pearl's father. He is very similar to his son in look and character, as he's always in a rush. He is probably a good friend of the unknown father of the player character. Just like the player and his rival did, he also got to a lake with the player's father and was also attacked by wild Pokémon.
Riley made his first appearance in the anime in DP110. He is a Trainer specializing in Steel-types and is currently staying at Iron Island for the study of said type. Due to the strange behaviour of the Steel-types on the island due to Team Galactic, he decided to investigate it with the help of his Lucario. In DP111 it was revealed that Riley is training to be a wave guardian with his Lucario and as such he is able to use aura effectively. He is able to create shields with his aura as demonstrated in the episode. After Team Galactic put bombs under the island after their plan was foiled, Riley and his Lucario trapped the explosion using their aura and sent it away from the island. Later he decided to help Professor Karashina in the study of the island so that he could know it better.