Byleth:

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Game - Fire Emblem: Three Houses

Attributes:
Archetype: Fast, Long-Range Zoner.

Weight: Byleth is a middleweight, clocking in at 98.
Walk: Byleth walks fast, clocking in at a walk of 6.
Initial Dash: Byleth has a very fast initial dash, clocking in at 1.45, however he has high traction so it doesn't go very far.
Run: Byleth has an above average run speed, clocking in at 10.3
Air: Byleth has very good air speed and physics, with an air speed of 8.5
Air Accel: Byleth has very good air speed and physics, giving him an air acceleration of 2.9.
Air Decel: Byleth has very good air speed and physics, giving him an air deceleration of -0.3.
Fall: Byleth has a high fall speed, clocking in at 15.
Fast Fall: Byleth has an average fast fall speed, clocking in at 20.

A spoilery sidenote
Edelgard, Dimitri and Claude are all in their pre timeskip outfits. Claude can still become a Wyvern Rider before the timeskip and his proficiencies are built to allow this. The Noble Lance, Cunning Bow and Victorious Axe are all their Fire Emblem Heroes weapons. They are still able to work together. They have not killed each other just yet.

Sword of the Creator: Byleth’s Creator Sword has two different states of being, being inert and sublime. The Sublime Creator Sword glows orange, deals more damage and can extend in an almost whiplike fashion, however the Sublime effect only has 20 uses. Once these uses run out, Byleth will gradually regenerate them all after about 15 seconds, however, whilst using the Sublime Creator Sword, if Byleth does not get hit nor take any action outside of movement for a total of three seconds, Byleth will regenerate 5 uses of the Sublime Creator Sword. Pressing Attack + Special will "activate" the Creator Sword and they can freely choose to activate and deactivate it in the air or ground.

Ground Attacks:
Jab - Byleth does a punch, a mule kick and then an uppercut with the Creator Sword. If Byleth uses the Sublime Creator Sword, this move will consume a single use and deal more damage on the final hit of the move. Based on Byleth's proficiency in Brawling.
Forward Tilt - Byleth does a slash forward, akin to Ike's forward tilt but faster. If Byleth uses the Sublime Creator Sword, this move will consume a single use, extend slightly and deal more damage.
Up Tilt - Byleth does a slash upwards, akin to SSBU Link's up tilt but a bit faster. If Byleth uses the Sublime Creator Sword, this move will consume a single use, extend slightly and deal more damage.
Down Tilt - Byleth does a kick. Has a really solid amount of combo potential.
Forward Smash
Inert - Wrath Strike: Byleth does a powerful outward slash whilst stepping forward. Byleth's strongest inert normal.
Sublime - Windsweep: Byleth does a powerful forward slash whilst fully extending the sword. This move consumes 4 uses of the Sublime Creator Sword and completely nullifies all attacks bar shield. Based on the B Skill from Fire Emblem: Heroes
Up Smash
Inert: Byleth does an upwards slash akin to Hero's up tilt in SSBU. A really solid anti air move.
Sublime - Grounder: Byleth does two slashes with the fully extended Sublime Creator Sword. This move consumes 3 uses of the Sublime Creator Sword and is an exceptionally powerful anti air move.
Down Smash
Inert: Byleth does a single spinning slash that hits both sides of him twice. Solid for covering rolls and tech options.
Sublime - Sunder: Byleth extends the Sublime Creator Sword and slashes both sides of him before retracting the Sublime Creator Sword. This move consumes 4 uses of the Sublime Creator Sword and the move's first hit combos into the second hit, which itself kills extremely early.

Aerial Attacks:
Neutral Air
Inert: Byleth swings the Sword of the Creator all around him akin to Ike's neutral air. Works well as a combo starter.
Sublime: Byleth swings the Sublime Creator Sword all around him, fully extending the sword. This move has an insane amount of range and starts combos. Consumes 2 uses of the Sublime Creator Sword.
Forward Air
Inert: Byleth swings the Sword of the Creator forward and upwards backhanded. Can end combos well and space well.
Sublime: Byleth swings the Sublime Creator Sword forward and upwards backhanded, fully extended. This move is a solid spacing tool and it consumes 2 uses of the Sublime Creator Sword
Up Air
Inert: Byleth stabs the Sword of the Creator upwards. Is active for an insane amount of time and can start combos with the late hit and kill with the early hit.
Sublime: Byleth swings the Sublime Creator Sword above him in a half-moon arc, fully extended. The move juggles insanely well and it consumes 2 uses of the Sublime Creator Sword.
Back Air
Inert: Byleth swings the Sword of the Creator behind him and downwards. Byleth's best non-Sublime aerial kill move.
Sublime: Byleth swings the Sublime Creator Sword behind him and downwards, fully extended. The move kills extremely early and it consumes 2 uses of the Sublime Creator Sword.
Down Air
Inert: Byleth swings the Sword of the Creator in a downwards half moon. The move meteor smashes, albeit not amazingly strongly.
Sublime: Byleth swings the Sublime Creator Sword down, fully extending it. The move meteor smashes and it consumes 3 uses of the Sublime Creator Sword.

Grabs and Throws:
Grab - Byleth grabs the opponent with his free hand. His pummel involves him bashing his opponent with the hilt of the Creator Sword.
Forward Throw - Byleth shouldertackles the opponent and knocks them away. Byleth's primary kill throw.
Back Throw - Byleth trips the opponent over his foot behind him. This throw leaves the opponent in a tech situation.
Up Throw - Byleth slashes the opponent upwards. Byleth's primary combo throw.
Down Throw - Byleth slams the opponent down and drives them into the ground. Sends at a far angle that can allow for Sublime normal followups. Based on Dimitri's attack against Flame Emperor soldiers.

Special Moves:
Neutral Special - Ruptured Sky/Sublime Heaven:
Inert - Ruptured Sky: Byleth creates a shockwave with his sword that only extends to the tip of his sword. It can be angled up or down if the button is pressed during the startup. Decent range, sends at a decent angle for continuing combos, and it also deals more damage to aerial opponents as a reference to it's effectiveness vs. Dragons.
Sublime - Sublime Heaven: Byleth extends the Sword of the Creator into it's whip form and creates a similar shockwave. If the move hits, Byleth will chain two more hits with the Sublime Creator Sword before finishing with a more powerful shockwave. Consumes four uses of the Sublime Creator Sword.

Side Special - Azure Moon: On both the ground and in the air, Dimitri will charge forward at the opponent using his Noble Lance. If the button is held, Dimitri will then hit his opponent towards Byleth, akin to how Waluigi’s Piranha Plant works, whereas if the button is not held, Dimitri will simply knock the opponent back powerfully. Dimitri can be hit, and he has around about 12HP. Depleting Dimitri’s HP will cause him to retreat and will put the move on cooldown for around 10 seconds.

Up Special - Verdant Wind: On the ground, Claude appears grounded and aims at the nearest opponent, whilst in the air, Claude flies in on his Wyvern, picks up Byleth by the hand and tosses Byleth above Claude before aiming at the nearest opponent with his Cunning Bow. The projectile will aim at where the nearest opponent was, akin to Palutena’s autoreticle, but it will not home in on the opponent. If no opponents are in range, Claude will look puzzled and disappear. Hitting Claude on the ground will cause him to retreat and puts the move on cool down for 6 seconds. Claude is immune in the air (cause he’s flying, and flying units avoid everything lol).

Down Special - Crimson Flower: On the ground, Edelgard will appear and assume a defensive stance next to Byleth. If any opponent attacks Byleth whilst Edelgard is next to him. Edelgard will swing her Victorious Axe and create a shockwave of flames that travels extremely quickly and does a high amount of damage. In the air, Edelgard will swing her axe and begin descending, creating a large shockwave upon impact. She can mix up the angles of her descent depending on which direction Byleth is holding, going from 45 degrees if the Byleth player holds forward, to 70 degrees if no direction is held, to 90 degrees of the Byleth player holds backwards.

Final Smash: Fódlan Gambit
Byleth slashes at the opponent before the scene transitions into a cutscene, where Edelgard, Dimitri, Claude and the rest of the Black Eagles, Blue Lions and Golden Deer all charge at the opponents caught in the Final Smash, all being led by Byleth. This final smash does massive damage and kills opponents extremely early.

Victory and Taunts:
UTaunt: Byleth swings his sword around him and says "Let the lesson begin!"
DTaunt: Byleth readies his sword in a two handed grip and says "Allow me to demonstrate!"
STaunt: Byleth appears with Sothis behind him and makes a thinking pose.

Victory 1: Byleth appears with Edelgard, the two make a battle ready stance whilst Edelgard is pointing the Victorious Axe at the camera whilst saying "You're weak!"
Victory 2: Claude flies in and picks up Byleth, putting them on his Wyvern with them, whilst he aims the Cunning Bow at the camera and says "Hold a grudge if you must!"
Victory 3: Byleth appears with Dimitri whilst Dimitri makes a defensive stance protecting Byleth and points the Noble Lance at the camera whilst saying "This battle is over!".