Ein

There's nothing particularly crazy about Ein, except for the fact that he and Hayate are the same person. Ein was the name Hayate used during the time of DOA2, when he had incurred amnesia at the hands of the Epsilon Project and had trained with Hitomi and her father at their karate dojo. Between DOA2 and DOA3, Ein had his memory restored, and thus he reappeared as the ninja Hayate in DOA3. Ein's appearance in DOA4 is thus apparently intended to portray Hayate as he was back in the DOA2 era. Thus, Ein is not going to have any of the moves that Hayate of DOA4 does, so being skillful with one character won't transfer over to the other; in fact, Ein's moves are more closely related to those of Hitomi than anyone else.

With all that said, Ein has no particularly spectacular abilities or moves to comment on. Some of his more interesting moves include the Raigyu (d/b, K, f, P, P) and the Kairai (d/b, K, f, P, d, K) are useful to try and vary the heights of your attacks.

Gen Fu

Gen Fu is quite similar to Eliot (due to their master/student relationship), relying on quick strikes with some odd syncopations to throw defenders off their game. He doesn't have a lot of particularly unique features, although he can, of course, be brutally effective in the right hands. One of the major differences between him and Eliot is that Gen Fu's throws usually result in pure damage, whereas Eliot's throws are more finesse moves, resulting in Eliot standing behind the opponent and setting him up for more combo-based damage.

Like Bayman, Leon relies on holds and throws to deal the bulk of his damage.

Leon

Although Leon is a different character than Bayman, they are quite similar in execution, mostly due to their similar emphasis on bruising punches and combo throws to deal damage. Leon is actually a bit more limited in gameplay for skilled players than Bayman, if only because he has far fewer options to counter incoming attacks.

Spartan 458 ("Nicole")

While an interesting character, Nicole is decidedly less full-featured than most of the characters in the game. Featuring only 56 moves in her sparring list (whereas most characters have 80 or 90), she's not going to be as versatile as the other characters you'll have unlocked by the time you finally get around to beating the game with Helena (which is the prerequisite for unlocking Nicole). She's particularly deficient in the area of kicks, since almost all of her main attacks involve punches. While she does have an amusing throw (the Plasma Grenade Stick), she's otherwise fairly generic. With practice you might be able to put her to good use, but apart from novelty, there's little to recommend her over many of the other more well-rounded characters in the game.

Tengu

Although Tengu was once the dreaded end boss of Dead or Alive, he's somewhat less threatening in DOA4. He can be unlocked if you manage to beat the game with every character in Time Attack mode, which is difficult due to the puissance of Alpha-152.

When you do manage to bust out with Tengu, you'll find that he has a fairly standard suite of attacks. He's not powerful because his moves are particularly threatening, but rather mostly because of the fact that his animations are surprisingly unique and his character model is bizarre enough to be difficult to adapt to, which is compounded by the fact that he's barely ever used online. (As of this writing, he's used twice for every hundred times someone selects Ryu, but that number will increase as more people unlock him.)

If you're going to try and use Tengu online, take advantage of his bizarreness by using his unique moves, such as the Koboshi-Inago (u/b, P, P), which sends Tengu flying into a backflip before reversing direction and slamming into his opponent with a body slam. If you're looking for a way to confuse your opponents, try his unnamed flight maneuver (u/f, P, P, P). It unfortunately can't be melded into an attack, but you can use a normal throw attack on the end of it to perform his Jodo-Okuri throw.