Does your opponent have to attack a monster you summon with A Hero Emerges?
No, not at all. When the defending player summons a monster
after an attack has been declared, a 'replay' occurs. The attacking
monster has to choose whether to continue its attack or discontinue
it altogether and forfeit its attack that turn.
If it continues the attack, it can be against a different
target. If the monster was making a direct attack, and a stronger
monster was summoned, then the attacker will most likely
discontinue his attack. He can't continue as a direct attack, but
is not forced to attack the newly summoned monster either.