PokeMaster could be really anyone out there, and he doesn't have to be from this site.
For example, a player can name himself 'Batman', but that doesn't mean he is Batman.
There's no real way to find a specific individual on the PSS unfortunately.
However, by adding a person to your friends list, you will know whenever this person is online, and you should see him on the top row of your bottom screen.
Hope I helped!