Similar word to underachiever:

also-ran, bankrupt, beat, born loser, bum, castaway, deadbeat, defaulter, derelict, disappointment, dud, flop, good-for-nothing, has-been, incompetent, insolvent, loafer, loser, lumpy, might-have-been, moocher, no-good, nobody, nonperformer, prodigal, turkey, underachiever, washout