"We Sheikah have served the royalty of Hyrule from generation to generation as attendants."
Impa is one of the most recurring characters within the Legend of Zelda series, only behind Link, Zelda, and Ganon themselves. In the early titles, she was depicted as a nursemaid and caretaker, while in the more recent installments she is a capable warrior and protector, as well as being the last member of the Sheikah tribe (of which Zelda took as a disguise in the form of Sheik). In Ocarina of Time, she was the sage of shadow. In Skyward Sword, she protected Zelda when Link was unable.
While the Legend of Zelda series most likely already has 6 characters in Crusade (Link, Zelda, Ganondorf, Child Link, Tingle, Sheik), I believe Impa is significant enough to earn a spot on the roster.
How would she play? As a member of the Sheikah tribe, she is very quick and nimble, able to weave in and out of combat with ease. She also carries a large sword from her Hyrule Warriors incarnation, giving her a large amount of range and power to back up that speed. Her disadvantage lies in the startup time to initiate a combo, as well as her relative lightweight compared to other heavy hitters. Once she connects, though, you're in for a rough time.
Combo: Quick Slashes
Impa makes a series of 2 quick slashes with her sword. It comes out a bit slow, but there's enough hitstun to follow up with another move.
Side Tilt: Spinning Smash
Impa leans back, then leaps into the air, doing some sort of cartwheel before slamming her sword down in front of her. Powerful, but takes a while to come out. It also hits opponents directly above her.
Up Tilt: Upward Swing
Impa swings her sword high above her head. It covers a lot of distance and is a good starting move for a potential combo.
Down Tilt: Sweeping Slash
Impa sweeps her sword low to the ground, knocking enemies away a short distance. While not as powerful as her other attacks, it is great for starting a combo off of.
Dash Attack: Sliding Kick
Impa slides forward, pushing enemies to the end of her path. She can press A again at the end of the slide to perform a kick forward, hitting those same enemies that got pushed and knocking them away. This kick is optional, so she can instead choose to simply get up - there's not enough time to follow up, but it allows a bit of mindgames with her opponents.
Side Smash: Iaido Slash
After charging, Impa makes a single slashing motion with her sword, taking it out of the sheath, swinging it, and returning it in a single motion. Extremely fast after the charging animation, as well as powerful. It takes a moment to reach that charging animation, however.
Up Smash: Shadow Orb
After charging, Impa, spins in a circle in place, summoning a shadow orb above her head. Enemies that get caught in it stay stuck in it momentarily until it explodes, causing a huge amount of knockback.
Down Smash: Shadow Blast
After charging, Impa leaps into the air and slams the ground, causing a burst of shadow to emanate all around her, knocking away enemies.
Neutral Aerial: Blade Spin
Impa performs a quick series of two spins in the air with her sword out, hitting a couple times before ejecting.
Forward Aerial: Side Slash
Impa takes her sword and performs a horizontal slash in front of her. It comes out fairly quickly compared to her other moves, and thus is an ideal follow up.
Up Aerial: Upward Thrust
Impa thrusts her sword upward. While it takes a moment to come out, it is powerful enough to get early KOs if used properly.
Back Aerial: Double Kick
Very similarly to Link, Impa performs a series of two kicks behind her. This is one of her fastest moves, though its power suffers as a result.
Down Aerial: Blade Slam
Impa puts her sword under her, and charges towards the ground. It goes through opponents, knocking them away.
Forward Throw: Shadow Slashes
Impa performs a series of 4 quick cuts with her sword, with the last one knocking away the opponent. These cuts have a shadowy trail visual associated with them.
Up Throw: Shadow Eruption
Impa turns around, and a wave of shadow erupts under the opponent, knocking him/her airborne.
Back Throw: Roundhouse Kick
Impa tosses her opponent behind her, and does a spinning kick to knock him/her away.
Down Throw: Shadow Daggers
Impa puts her hand up, and a series of shadow blades form above the opponent, slaming to the ground. This causes a series of multiple hits before ejecting the opponent upward slightly, allowing a potential follow up.
Neutral Special: Sheikah Barrier
Impa puts her hands out, and a blue barrier appears in front of her. This barrier blocks all attacks and reflects projectiles as long as the button is held. She is still vulnerable from behind, and it does eventually break after some abuse.
Side Special: Ninja Dash
A chargable special, Impa stands in place before dashing forward, knocking away enemies in the way. It covers a lot of distance with little charge, and a full charge will send her accross the map.
Up Special: Vanish
Identical to Sheik's, she uses a deku nut on the ground (self in the air?) and appears a short distance elsewhere.
Down Special: Nocturne of Shadow
Impa puts her mouth to her hand, and whistles the Nocturne of Shadow. A vortex of shadow appears in front of her, pulling opponents into it while dealing rapid damage. The vortex lasts enough time (~5-10 seconds) to follow up from it.
Final Smash: Bongo Bongo
A well appears, and a large invisible creature comes out of it. The shadow is visible, however. This creature grabs enemies and throws them around, dealing a large amount of damage and knockback. After a set amount of time, it returns to the well and Impa seals it shut.