IN-GAME REWARDS?

[bun bun]

Do you get rewards for completing lets say... S+ RANK under 2 hours in hardcore mode?

[James.Havoc]

No. Instead you'll receive points for completing challenges like that. You can then use those points in the in-game shop to buy outfits, Infinity weapons, coins to boost your damage, heal you or raise your defense against attacks as well as other helpfull stuff.

[Big Boy]

Not directly. At least not anything worth for gameplay value.

Finishing every difficulty will unlock certain models and concept arts. But no in-game items like costumes, weapons, etc. You buy them from the Points Shop, which unlocks after finishing the game for the first time on any difficulty.

Finishing in Hardcore mode will get you the "Veteran" achievement, which is worth "1200 points" to use in the Points Shop and spend however you wish.

Finishing the game on Hardcore will also unlock Nightmare difficulty, which upon finishing will earn you 3000 points; and finishing on Nightmare will unlock Inferno difficulty, which finishing is worth 8000 points.

You can use these points to purchase in-game items and costumes that you want.

Game has no S+ rank. Only S, if you can finish under 2 hours with 5 saves or less.