The play does mention "hemlock" as a poisonous plant used for suicide.
* In Act 5, Scene 5, Lady Macbeth, consumed by guilt and madness, speaks of "the poisoned chalice" and asks "Sleep no more". The implication is that she contemplates using hemlock to end her life.
* It's not explicitly stated that she actually takes hemlock, but the implication is strong.
Therefore, hemlock is not a character, but a symbol of death and poison in the play.