Sir. Aaron appears in the 8th Pokémon movie, Lucario and the Mystery of Mew. In the movie he is shown in flashbacks as he is dead prior to the start of the story. He is the mentor and best friend of the Lucario that was sealed in the staff of the guardian. Sir. Aaron is a master of the aura and he passed his skills onto Lucario. The Lucario in the movie is even stronger with the aura than the usual variety and uses it more like Aaron would have due to Aaron's influence.
   Sir. Aaron makes a more disguised appearance perhaps as a descendant of himself in the anime as Riley, when ash and co. travel to the Iron Island and find him trying to get to the bottom of the steel type Pokémon's rampage on the island. He says in this that he is training alongside Lucario to become an Aura Guardian which is his title in the movie.
   He appears a third time also as Riley in the Diamond, Pearl, and Platinum versions of the Pokémon main series of games. He is on Iron Island here as well and partners with you for part of the cave using a Lucario and he heals you after every fight. He gives you a Riolu egg after you help him get through to the end of the cave. Riolu is the pre-evolution of Lucario.
   His appearance is tall with black some-what spiky hair and a black hat with a blue strip in the middle and a symbol that is presumably the royal insignia as he is show as a royal knight and lives at the castle. His normal outfit is blue and grey with a long black cape. He carries a staff with him that in the movie acts much like a pokeball for Lucario as he gets trapped inside until ash's time about 1000 or more years later. The staff is made of wood and has a diamond on top with two metal pieces resembling the feeler like things on the side of Riolu and Lucario's heads.
   As Riley his appearance changes a bit. He is still tall with black spiky hair but the hat is blue now with a black strip and no symbol. His outfit is mostly blue and he does not have a cape. The staff is not present either.
   As mentioned earlier they both use the Aura which is the living essence inside of every creature. It can't be seen by most people but those that can sense it and control its power were at one time common in the world of Pokemon. Hence Sir. Aaron and Riley.

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Pretty good review about Aaron, but are you sure Riley actually is him or are you just guessing?

He has been speculated to be related to him and or a dependent. They look a lot alike so I put him with it.