This build is a 4th Edition portrayal of an ancient literary/cinematographic archetype: the swashbuckler! Unlike most swashbucklers, though, this one does not try to steal all glory for himself; no, like Cyrano de Bergerac, he stays in the shadows while another climbs up to kiss the sweet rose. It has some nifty tricks to take advantage of the Rakish Swashbuckler's 11th-level feature Vexing Foe, and of the Rakish Swashbuckler's 11th-level encounter attack power Abashing Stab.
On to the build itself:
Race: I chose Human because of the extra Feat - not to mention aesthetic reasons. 
However, a Drow - or any other race with bonuses to both Dexterity and
Charisma, like a Halfling - would also work very well. And, as
Seifalmasy pointed out, Half-Elf would also be an excellent choice: not
only because of the bonus to Constitution, but also because of the
Dilettante racial feature and the new Versatile Master Feat, which would
be more useful than the Human's third At-Will.
Class: Cunning Bard (Rogue)/Rakish Swashbuckler.
1st-Level Attributes: STR 10, DEX 16, CON 12, INT 10, WIS 10, CHA 18. Dexterity and Charisma will be increased at every opportunity.
Skills: Acrobatics, Arcana, Athletics, Bluff, Diplomacy, Insight, Streetwise.
Feats: 1st - Weapon Proficiency (Rapier), Sly Dodger (Bluff); 2nd - Melee Training (Charisma); 4th - Focused Expertise (Rapier); 6th - Nimble Blade; 8th - Weapon Focus (Light Blades); 10th - Action Surge*; 11th - Shield Specialization (Light); 12th - Improved Cunning; 14th - Disheartening Presence; 16th - Paragon Defenses; 18th - Light Blade Precision; 20th - Cull the Weak
* For nonhuman Dashing Swordsmen, another Feat should be taken.
At-Will Powers: Guiding Strike, Misdirected Mark, Vicious Mockery
Encounter Powers: 1st - Inspiring Refrain; 3rd - Charger's Call/Cunning Ferocity; 7th - Timely Distraction; 13th - Charger's Call/Cunning Ferocity Couplet of Deceptive Weakness; 17th - Inspiring Refrain Word of Vulnerability
Daily Powers: 1st - Echoes of the Guardian; 5th - Word of Mystic Warding; 9th - Satire of Bravery
Utility Powers: 2nd - Song of Courage/Concerted Effort; 6th - Allegro/Synchronicity; 10th - Savior's Song/Song of Recovery; 16th - Allied Rhythm/Elegy of the Undefeated
Relevant Equipment: Rapier, Hide Armor, Light Shield. Since so many of the powers have the Implement keyword instead of the Weapon keyword, a Songblade - either Piercing or Lilting - is required for this build to work. Boots of the Fencing Master are also particularly appropriate.
1st-Level Defenses (Human): AC 17, Fortitude 12, Reflex 16, Will 16.
Vexing Foe: With Misdirected Mark, you can have your target marked by an ally within 5 squares you; combined with Vexing Foe, that means the target will have a -4 penalty on attack rolls against you, and -2 against anyone else but his marker. If you first hit your target with Word of Mystic Warding, and then use Misdirected Mark, he will be dealt damage whenever he moves closer to his marker - hopefully while also provoking opportunity attacks from all adjacent enemies (which may include you). (Once per day, you can use Echoes of the Guardian instead of Misdirected Mark.)
Abashing Stab: With Abashing Stab, you mark your target; combined with Vexing Foe, that means the target will grant combat advantage to all of your allies. You must first hit your target with Word of Vulnerability. Then, on the next round, you can use Abashing Stab and then spend an Action Point to immediately use Satire of Bravery. Whenever your target moves closer to you, he will take damage; meanwhile, all of your allies will deal additional damage on him.
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