The Heavy is an Infantry unit introduced in the 4.1 Patch.
The Heavy attacks with its trusty mini-gun Natascha. It has three distinct attacks: a horizontal row attack that can shoot over some units, an aimable column attack with splash damage along the edges, and an anti-air attack that also deals splash damage. Each attack has its own ammunition pool, and all attacks have Armor piercing.
The Heavy has high Health for early levels and gains Armor at Rank 5. It also has 60% Crushing Resistance and is immune to Cold Environment and Freeze effects. The combination of durability, armor-piercing attacks, and access to all three weapon abilities makes the Heavy a powerful front-line unit at the level it is unlocked.
The Heavy's weapon is named Natascha, in reference to the Natascha, one of the TF2's many miniguns. Interestingly, the minigun depicted is the Heavy's stock minigun, which he affectionately names Sasha, even to the embarrassing point that the Heavy needs to buy Sasha a bigger bed.
When the Heavy uses the Get In Line attack, you can hear a laughing line if your sounds are loud enough. This is heard not from the game itself, but the Meet the Heavy during the final scene.