Whenever you scry, put a +1/+1 counter on target creature.
: Scry 2.
Arwen turns every scry into a +1/+1 counter, so you build a deck flooded with scry effects to grow your board while smoothing draws. You curve out cheap scry enablers, pile counters onto a threat or a wide board, and close with proliferate payoffs, counter synergies, or a big buffed attacker. Her built-in : Scry 2 gives a reliable mana sink in the late game.
Cheap two-mana commander that converts a ubiquitous keyword (scry) into permanent board growth
Scry is everywhere in Simic, making enablers plentiful and budget-friendly
Each separate scry trigger gives a counter, rewarding multiple small scries over one big one
Doubles as card selection engine, so consistency and board development happen simultaneously
Relies on her staying on the battlefield; she's a fragile 2-mana body and a removal magnet
Counters do nothing without evasion or proliferate payoffs to convert them to wins
Multiple small-scry effects matter more than scry quantity, which can be unintuitive to build
Vulnerable to board wipes that erase all your invested counters at once