Odd that the trigger isn't optional, so an opponent can bait him into removal zone when you're at 14 life or so.

The "in your shadow" ability is problematic. In general it has the issue of being highly complex - not a problem on this rare, but I imagine you want to use it on commons as well. Just look at the potential for complexity by hitting you with a Lightning Bolt during combat and sending you from even life total to odd.

Additionally on this specific card the static bump to power and toughness actually interacts negatively with the "in your shadow" ability since it can bump a thief out of the shadow.

Having a sneaky ability is cool and all, but this seems like a dead end. I personally wouldn't oppose shadow before the "in your shadow" ability.

(Flavorwise "in your shadow" is also weird. Being in someone's shadow has the connotation of being insignificant rather than being sneaky. Also it doesn't work as an idiom for stalking/lurking since your own creature shouldn't stalk you, but rather your opponent - or their creatures.)

@Mal, @SecretInfilrator both good points, thanks for the feedback! Now that you mention it, shadow should flavorfully represent being in your opponent's shadow or that of their creatures. Maybe "~ is in shadow as long as your opponents don't control any creatures with fewer power."