There exist several different Reesa cards that are used for Enhance+. Each different Enhance+ Reesa card has a different rarity, and is only eligible to impart the Enhance+ bonus to a pirate of the same rarity.
To use a Reesa for Enhance+, select a max level crown pirate that is not already enhanced. and select a Reesa of the corresponding rarity as the jewel pirate. Note that the Reesa can be any level for Enhance+, unlike the max level requirement for the jewel as well, unless the crown pirate is also an Enhance+ Reesa.