Granblue Fantasy (or gbf for short) is an RPG developed by Cygames. It's available in Japanese for iOS and Android, and available on web/mobile browsers in English and Japanese.

The game is great, and I recommend it highly to people who are looking for a challenge in terms of grinding in games!
The game has a very good f2p option.
The game also makes use of a gacha system common among mobage (japanese mobile games) where you use crystals in order to summon in 10-part draws, or solo draws.
The game also has an anime with the same title.

what's the plot?

You (commonly voiced as "danchou" or written with the nickname you chose) receive a letter from your father who is allegedly waiting for you on Esta Lucia, the island of astrals of legends - therefore its existence is unknown.

Right before you set out from your hometown Zinkenstill, you encounter Lyria, a girl who is on the run of the Erste Empire. An incident leads both of your fate to be connected and you embark on a journey with her and her guardian Katalina to find Esta Lucia.

The characters you can recruit for your party won't affect the main story and are treated as fellow crew members which you collected along the way.

It's also important to notice that you can actually play the game in your browser at game.granbluefantasy.jp.