Hello, so I am playing Pokemon platinum and I want a ditto to breed my starter and ev train it before the league. But I cant get it in the trophy garden. Also the man doesnt say anything about any Pokemon sneaking into his garden. I hope you can help me.
In order to catch a Ditto in Trophy Garden, you actually need to get the National Dex. So, in order to get the National Dex, you need to complete the Regional Dex by seeing every Pokemon. After this, go to Professor Rowan and he will give you the National Dex. Then, go to Mr. Backlot's Mansion. Now go to his office and SAVE. This will allow you to reset in case Mr. Backlot doesn't see a Ditto. Reset until he says he saw a Ditto in the Trophy Garden or his backyard. Now just keep walking in the tall grass until a Ditto appears. I would honestly do this after the League though. You don't need to EV train it before the league because you can just overlevel. And you need to trade in order to complete the Regional Dex.
You have to have the national dex for the man to talk about Pokémon in the Trophy Garden
Because of this, you can only get one by trading with a friend.