The Heat Riser buff gives you an extra 10% boost to both your damage and defense.
For example, when you do j.C, instead of dealing 800 damage you'll deal 880. Or instead of dealing 900 damage with 5C, you'll deal 990 damage.
Another example for when you take damage instead of dealing it, instead of taking 900 damage from Narukami's 5C, you'll instead take 818 damage when the buff is active.
The defense buff also works in tandem with the awakening defense buff, making you not only somewhat tanky, but also incredibly dangerous with your awakening supers on deck so you can deal more damage in your combos.
This has one drawback though, if by any chance you lose all 4 cards and get persona broken, you lose the buff. Meaning you'll have to reapply the buff again if you want your damage and defense buff back. So make sure to use your persona moves wisely so you don't lose your buff immediately after getting it.