Reasonable Doubt

Mana Leak with an investigative rider stapled on. The counter half is soft permission in its purest form: a two-mana tax that punishes an early curve-out but decays into nothing once the opponent can spare the extra mana. What the design bolts on is a clause pulling in the opposite direction. Countermagic wants you to hold up mana and act on your opponent's turn; the suspect half is proactive, a downgrade you slap on a creature to strip its ability to block and give it menace. The catch that makes the pairing honest is that both halves live on the same instant, and the card only casts when there is a spell to counter. You cannot fish the suspect clause out on an empty stack. So the reach the second clause offers is conditional on the opponent giving you a target: when they cast the threat you wanted to Mana Leak, you get to counter it (or force the tax) and simultaneously pop suspect on a blocker to clear a lane. When they stop casting, the whole card goes quiet, suspect included. That is the tension the design is built around, and the reason the "up to one target creature" is optional: on the many turns you fire it purely as a counter, the rider is a bonus you can decline, not a cost you have to pay.