He shows up on a computer on Mocktropica Island, there is a user profile, you can enter it and view him.
He has a recolored bronze sword similar to the Rusty Relic.
Pictures of him are seen on the same computer that has his profile on Mocktropica Island. They are assumed to be vacation pictures based on the titling and variety of scenery of different islands.
Some have speculated that his username may be SteveCreator, which would imply (but not necessarily confirm) that it was owned by a Creator named "Steve." The account's name is Giant Walker (an ordinary Poptropica name rather than a custom one that Creators tend to have).
You are able to get the skull mask of Test123 in Legendary Swords, in the same cave you get the Rusty Relic.