I have created a Pokemon themed app , and I'm going to upload it to Google Store but I have some doubts. I have two versions of the app , one free and another "premium" which costs 2$ , also I want to introduce ads in the app in order to get some money . Is this legal ? Can Pokemon turn down my app ? Do I need to specify something in the description in order to tell Pokemon that the content I use don't belong to me ?
Your app is a simple case of copyright infringement. All the Pokémon are copyrighted, the lettering and names are also protect by trademarks.
Trying to claim fair use will be outright impossible: you'll use huge portions of the individually protected Pokemon (the iconic ones like Pikachu) and you are usurping a market they are already in. They have given licenses for apps (Pokemon Go). Pokemon are artistic and some form of fiction. The last straw might be if you'd do a rather obvious parody, but even then, I see no way to show Fair Use with what you stated. No disclaimer can change that, and publishing your work might open you to a huge lawsuit with damages for each individually protected Pokemon you infringed on. With between 750 $ minimum and 150,000 $ absolute upper limit per infringed item (last is for willful infringement), you don't want to infringe on Pokemon, as you could be very easily liable for a number in the 6 to 9 digits! Even if Nintendo might only try to get the statutory damages for all the 900 Pokémon, that is a number of at least 675,4000 $.
And that's before looking at Trademarks. Pikachu has about 6 live word marks and there are 111 different
Pokémon trademarks filed (some expired or dead)!