If you possess a Spirit that matches the players rarity (e.g. 7★) and category (e.g. [Ex]), you can use it for a Limit Break.
With every Limit Break the players maximum level rises by 5 levels.
You can use up to 4 Limit Breaks on any one player, so this gives a total of 20 levels extra maximum.
You can get Spirits by selling off players.
The Spirit you get will match the player you sold in terms of rarity and category.
*You need a Spirit with matching rarity and category for each Limit Break.
5★ player → 5★ Spirit
7★ [Ex] player → 7★ [Ex] Spirit
*You can't get Spirits for transferred players.
As an exception to this rule, you can get Spirits for transferred players if they were in your possession before maintenance on December 20th 2016.
*You can transfer players who have undergone a Limit Break but the Limit Break status will be reset.