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A few of my suggestions, just in case:

Pandemonium!

Duke Nukem 1.

Rayman Forever. Straightforward and beautiful platformer, but really tough.

Tyrian 2000 might be worth a look, too. It's not a platformer, but it doesn't require much thought and is really good, not to mention free.

There's also Secret Maryo Chronicles, a free, open source clone of Super Mario World.
Post edited June 15, 2015 by InfraSuperman
Here's a suggestion that I haven't seen posted in this thread before:
Ys: The Oath in Felghana.

In terms of each criterion, here's how it fares:
1. This game can be pretty difficult. Normal is a good difficulty, and there are 5 other difficulty options to choose from.
2. Doesn't have much in terms of puzzle. Does have unskipable story sequences, unfortunately, but it isn't a huge epic either.
3. Many enemies take different approaches to deal with them. Due to the way the XP system works, even if you need to level up, it won't take long to reach the level the game expects you to be.
4. There is plenty of platforming; more than in any other Ys game to my knowledge.
5. Blood can be turned off, and even when you do fight human enemies, the graphics aren't particularly realistic.

The game is often referred to as an RPG, but I think that's misleading; it's really an action game that just happens to have elements commonly seen in RPGs. There isn't even any careful stat management or real resource management (your MP and Boost refill rather quickly).
Oh yes, and definitely check out Rayman Origins
she don't need the games, i'll play with her :)