This is one of those combinations that can have some mechanical synergy if you plan for it. Fighters really just need either Strength or Dexterity for their weapons of choice and espers need Intelligence to make their powers work to their fullest potential. In those terms, it’s comparable to a class like the herald or wielder in a single-class build. Flavor-wise, this works as well as you care to make it work. The general concepts of “highly-trained warrior” and “person with psionic abilities” leaves a lot of room for interpretation, and it is no stretch at all to see those coexisting in the same person. More levels of fighter gives you a trained warrior with some minor psychic abilities they can draw on, possibly in combat, possibly mostly outside of it. More levels of esper gives you a powerful psychic who knows what to do with a weapon. An equal mix of the two gives you an integrated character that can blend their psychic powers and combat training into a seamless, formidable whole. [[Synergy Feats for Multiclassers#Esper w/ Fighter|Esper-Fighter chains and ideas]]