Elves' secret unit is the Unicorn. In the screen before entering the big tree, there will be an elf guarding a cave off to the right. If you fight him, you can enter and buy unicorns.
The humans have... some kind of sword knight guy. Outside the evil guy's mansion, there should be a bad guy guarding an angel statue. After you defeat him, a path will open up in the angel statue (you might have to interact with it or something, I forget) and you can go down to buy the sword guy.
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