Tetsuko Umezawa, Fugitive

Oracle Text

Creatures you control with power or toughness 1 or less can't be blocked.

Card Rulings

4/27/2018 Once a creature you control has been blocked, changing its power to 1 or less won’t cause it to become unblocked. Changing its toughness to 1 won’t cause it to become unblocked, and changing its toughness to less than 1 will cause it to die.