Haku-men's specialized Heat Gauge, also known as Magatama, is a unique characteristic of Haku-men.
Haku-men builds his gauge slowly overtime, if no action is performed. Landing attacks generate a portion of a Magatama (up to a max of 8 Magatama, or the equivilent of 100% Heat.) Countering attacks with his drive generates 1 Magatama per counter, except for 6D. Instant Blocking generates 1/3 of a Magatama. Cutting a projectile generates 1 Magatama.
However, all of his special moves consume Magatama. Here is a list of which move consumes what.
Gurren - 214A (1 Magatama)
Renka - 214B (2 Magatama)
Kishuu - 623A (1 Magatama)
---> - Enma - A (1 Magatama)
Zantetsu - 41236C (3 Magatama)
Tsubaki - j.214C (3 Magatama)
Hotaru - j.214B (2 Magatama)
Kokujin: Yukikaze - 236236D (4 Magatama)
Kokujin: Shippuu - 632146C (4 Magatama)
Kokujin Ougi: Mugen - 214214B (8 Magatama)
Aku Metsu - 28D (8 Magatama)
Now for how useful each for it's cost.
Gurren is a fantastic move for it's cost, and is used in many combos for it's wall bounce properties, outside of combos, it's use is very limited.
Renka is one of Haku-men's best uses of Magatama, fast, hit's low, and leads into two of his best combos. Renka can be cancelled at any time into Kishuu, so for his most essential combo, his Falling j.2C combo, it goes Renka (First Hit) > Kishuu > Enma.
Kishuu is a good move itself, providing limited invulnerability, but outside of combos it also has limited potential, especially with it's follow up Enma.
Zantetsu has lost some of it's wham in Continuum Shift, and is generally not used, the 3 Magatama has better uses elsewhere.
Tsubaki can be used from a hop cancel or in air, but it's generally not the best idea to try to start a combo with it. However, on counter hit, Tsubaki leads to an incredible amount of damage.
Hotaru is hands down the BEST use of Haku-men's Magatama outside of combos. On Fatal Counter, you can land combos of 5k+ damage, with ease, in most cases breaking 6k or 7k. During combos, Hotaru has very little use.
Kokujin Yukikaze, Haku-men's distortion counter, is very flashy, seems neat, but at it's cost, is not worth it. There are better uses for the gauge.
Kokujin Shippuu has almost no use outside of combos, and even in combos, very limited use.
Kokujin Ougi Mugen is a great follow up for 6D, if you know his guaranteed kill combos. Other than that, it's a gimmick that most avoid.
Aku Metsu is his astral, and well..it's an Astral. It's a waste of gauge guaranteed, but it looks cool, and that's why people use it to finish someone off. The real thing is, if you have this much gauge sitting around, you're doing something wrong.