you have to have all 8 gym badges
you need to do the Pokemon league
if your talking about the man in victory road he disapeers after u beat the league. but if your talking about the castle aboe the waterfall i ha ve no idea what u mean?
In canalave city, the old man inside the house that's in-between the pokecenter and the pokemart.
One of your Pokemon needs to have 9 ribbons. It doesn't have to be contest ribbons. You could get other ribbons by talking to the footprint man, telling a story to this lady in Sunyshore or win the Pokemon League (the pokemon has to be in your league team though).
You will need to unlock the National Dex and defeat the Pokemon League to make him move.
In Pokemon Diamond/Pearl, you find a old man at route 212 with a Leafeon.
you need to become Pokemon League Champion then help Celio on One Island to get his computer running
in etrna
there will be a man fishing in sunnyshore city
you give him a drink
in Eterna city
after you get more than for moves by the pokemon's level or using a hm or tm you can select one to delete. if it is an HM, then you need to go to canvalae city and go into this house with a old man in it he will ask you if u want to delete a move