NINJA GAIDEN Σ

Hard is the most difficult option available for the first playthrough of the game, though I would recommend strongly against this, as Normal is useful for learning the path through the game, as well as how to fight the many varieties of enemy and the bosses. If you have first played and beaten Normal, this is just the next step up and you will get used to the changes without too much trouble. For a first playthrough, it will feel like climbing a mountain, but it should still be possible if you stick with it enough and put in the time to learn enemy behaviour and the combat moves.Below is a list of changes that are made between Normal and Hard:

Enemies are tougher and do more damage to Ryu
All bosses will now have some variety of minion to accompany them, spawning a small group per 25% of health lost by the boss.
Ambushes (the kill challenges where Ryu faces large enemy groups) are 20 kills longer, now 80 killls, and some have had the enemy spawns changed.
Some enemies now appear earlier or more often, such as the Bast Fiends.
Fewer health items are given, with their spawns being replaced by Life of the Gods.
Some chests now contain bats or ghost fish, instead of an item
There are very few free Ninpo items (a few Elixirs of the Devil Way), they all must be bought from Muramasa.
Some items (like the Wooden Sword) are now golden scarab rewards, where they were previously able to be bought from Muramasa.
Ryu clones now appear as enemies in Chapter 7 and 15, wielding the Dragon Sword and Vigoorian Flail, as well as unlimited Ninpo and Ryu's moveset.
All healing and Ninpo items are now more expensive, by 500 for small items (Elixir of Spiritual Life, Elixir of the Devil Way), and 5000 for large ones (Great Spirit and Devil Elixirs, Talisman of Rebirth)

Beating Hard will unlock Very Hard and the Biker Costume for Rachel. as well as some new pieces of equipment along the way. In particular, beating the Alma boss fight will unlock the Azure Dragon Helm, which decreases Ryu's defence but makes him deal a lot more damage, which is by far the most useful piece of equipment.