Carl and Nirvana move separately, alternating their attacks to create deadly offensive mix-ups. Carl moves quickly, but has weaker attacks. Nirvana is slower, but hits like a freight train! The two fighters have different roles and must be used in concert to be effective.
Carl's ability lets him move Nirvana, while Nirvana's ability makes all of their Specials and Ultras more threatening while they have the opponent surrounded.

Strengths & Weaknesses

As long as they keep the opponent trapped between them, Carl and Nirvana exert incredible pressure due to the raw stats conferred by Nirvana's ability. However, it takes time and resources to maneuver the opponent between them.
They're far less threatening when out of position, and neither of them is particularly effective at long range. Opponents can temporarily disable Nirvana by "hitting" her with attacks, so Carl must be careful not to leave her too exposed!
Mechanics

Carl's Overdrive provides a safety net for Nirvana by letting him restore her functionality, protect her for its duration, and reposition her each turn. His Overdrive is cheaper and shorter than most in BlazBlue, since he usually only needs it to recover from a perilous situation.
Carl's Astral Heat, Deus Ex Machina, has excellent damage and defenses at a high cost; Nirvana is Sealed after the attack, removing her from the game! This risky Astral is great at closing games or protecting Carl in a moment of late-game desperation, but if it doesn't end the game immediately, it will leave Carl virtually helpless. He seldom uses it unless he's sure it will connect!