Skill 1: Unison Attack
For 4/5/6/7 seconds, whenever the nearest ally hero within 700 range attacks, you attack the same target with a ranged KI-burst for 40/50/60/70% of your base damage. No cast time, running/attacking is not interrupted.
Skill 2: Flash Kick
You teleport to the enemy and kick him in the direction of the nearest ally hero. 200/300/400/500 range fast pushback. The pushback ends when the enemy hits a tree or unit in the way.
Skill 3: Unison Formation
The next time you are 500/900/1100/1500 range apart from target ally hero, after 0.3 seconds, you teleport in front of him and stun all enemies in 250(?) range for 0.5/1/2/3 seconds.
Subskill 3: Forced Unison
If Unison Formation is active, you teleport instantly, regardless of range.
Skill 4: Ultimate Combination
The next time target ally hero uses a skill that is not passive or a toggle/autocast within 8 seconds, you use the skill, too. Any damage you would deal is reduced by 60/45/30%. Targets (ground/unit) stay the same. There is no mana cost for the borrowed skill. Range requirements for the borrowed skill are ignored.
Low cast range for the initial spell.
Playstyle: Obviously, pair up with another hero. Use Unison attack on fast-hitters like Nymphora or agi based heroes to kill single targets quickly. Use Flash Kick and Unison Formation in combination to help your ally catch up to the enemy and stun him additionally. You can also hide in the woods and have a stealther or someone with blink go into the enemy and teleport+stun in yourself. The Ultimate is .. well, go figure out fun stuff. It's possible to stealth yourself, cast Pyro's ultimate, Devourer-hook, kill yourself by copying Legionnaire's taunt, charge at the same time like Kraken or Zephyr, summon a Malphas ..
GFX thoughts: Skill 1+4 could have yellow/red glowy auras around both chars involved, the hit-animation from skill 1 could be a blue energy burst on the enemy (something like Starcraft's Corsairs). The position where you teleport to with skill 2 would be in a straight line with the ally behind the enemy.
(ally) --- (enemy)(you)
Balancing thoughts: Well yeah, the ultimate. Depending on other powerful skills, there could be some overkill combos.
Skill 1's damage, recast, duration and manacost can be tweaked heavily to increase or decrease the UF's performance to get him to a desired level.
Might want to add a damage component to skills 2 or 3, or a snare/silence to skill 2.