Hearthpwn

Doesn't get buffed by Unstable Evolution (I know it hasn't Echo but it works the same). Small Indie Company Kappa

Nice art.

I see some interssting synergy with Pickpocket,Spectral Cutlass, and Tess Greymane.

Very similar, but worse, than Questing Adventurer. Not sure if there will be enough echo synergy for this.

It may not fit into many constructed Rogue decks, but it's a strong card, especially if there are a lot of good echo cards. 3/5 is already a good stat line because a 5 health creature is difficult to deal with for most classes.

It might be a decent midgame minion for rogue. Definetely worth trying at least.

I wish this card was a neutral card

On the plus side, his statline is decent enough to be played on curve without the need to immediately activate his ability.

Card text is a worse version of Questing Adventurer.

However, this card is very different in how it works from Questing. The big difference between the two cards is that Questing is only really playable if you buff him on the same turn. You can't drop Questing Adventureas a vanilla 2/2 on turn 3. However, the Mistwraith being a 3/5 for 4 mana means you can drop it on curve, and have a decently high chance of it surviving.

Also, due to having both a high reward for playing Echo cards, and still being decent without the reward, I doubt you'd even need that many Echo cards to justify running Mistwraith. Even a deck that only ran two Cheap Shots and a Face Collector could use it. Sure, 2/3 of the time Mistwraith won't get a buff. But if you do the math, considering you'll usually play 2-4 echo cards at a time, Mistwraith still ends up being pretty good.

I will admit, however, that there is currently no viable Rogue dech archetype which has enough room to play minions that are "simply above average", as they're all heavily focused on synergy. Mistwraith could see play in a more minion-heavy Midrange deck, but considering how many spells Rogue naturally runs, it's doubtful there will be any room for minions played simply for stats.

If Echo gets more development this could be okay. 

Would be fun to combo with Sound the Bells on turn 10. 9/14

Problem is that Questing Adventurer gets buffed  by all cards including echo cards, so is a way better option if you want this effect.

Decent arena card but thats about it.

Well doesn't seem to be better than Questing Adventurer

And doesn't snowball as fast as Ethereal Arcanist
... or does it ?!

well I mean you can give it +5/+5 for 10 mana... but that doesn't seem like a very high tempo... Of course with Cheap Shot you probably also cleared the board a bit, but still kinda meh... though if you have loads of echoes in the deck you can give it +3/+3 or +5/+5 every turn and lose to any hard removal...