Man-Bat is Kirk Langstrom. He is an enemy of Batman.
He is a scientist who develops an extract intended to give humans the bat's sonar sense and tests the formula on himself because he is becoming deaf. While it works, it has a horrible side effect: it transforms him gradually into a hideous human bat.
He first clashes with Batman when he tries to steal the chemicals needed to reverse his transformation, but his control over the new animalistic instincts resulting from his bat-like state is tenuous, and Batman is forced to subdue him in order to administer the antidote.
He clashes several times with Batman in the Batcave (which he finds by following other bats through the network of caverns; he does not recall its location in human form, and he never knows Batman's secret identity), believing erroneously that his daughter is dead and that Batman is somehow responsible.
He begins to suspect that his cures missed an element of the serum, and he's losing control to his bat side. He believes he has slaughtered his own wife, son, and daughter.
However, both Kirk and Francine are shown to be alive in the "One Year Later" storyline.