When Luke settled on Bard as his class, he pondered long and hard about the Affinity he would choose. For starters, he removed Earth Affinity from his thinking; it should not be even taken into consideration as that affinity majorly contributes towards Defence. Though there are ways of playing with that Affinity Luke did not want to risk already made character.
Fire and Water are both playable with the Bard class. If he chooses Fire as his Affinity, he will go toward extreme offense, where his attack skills may acquire more damage. Meanwhile, Water will result in a versatile style of play in which his mobility and healing-type skills may gain a boost.
Though Luke despised the Wind Affinity since he died from a 'Wind Slash'. He had to concede that it is the best affinity for the Bard Class since it could offer an offensive as well as a versatile playstyle, as Wind affinity could result in a boost in his Attack and Movement Type skills.