Shukaku is the One-Tailed Beast sealed inside Gaara, and it's basically the source of both his immense power and his mental instability. The demon can fire off chunks of sand that function like kunai, making it a versatile weapon in combat. In the English dub, Shukaku gets this over-the-top insane cowboy voice that's pretty memorable.
As an Ichibi, Shukaku is genuinely powerful—powerful enough that Gaara struggles to fully control it even during the series. That lack of control is a big part of what drives a lot of Gaara's character arc early on.
Content compiled by AnimeList.moe from publicly available sources.
