Delighted Halfling

Creature — Halfling Citizen

: Add .

: Add one mana of any color. Spend this mana only to cast a legendary spell, and that spell can't be countered.

The legendary spell can't be countered if the mana produced by Delighted Halfling is spent to pay any portion of the spell's cost, even an additional cost or an alternative cost. This is true even if you pay an additional cost while casting a spell "without paying its mana cost."
If the legendary spell you cast this way is copied, the copy can be countered.