So this is the Taisia I'm building.

Char:

Start
Brien Fiend x 3
Kiridwen x 3
Taisia

Others:
Don't know for sure, I need some opinions.

Actions:

Too fast to see x 3
Misear Diplomassy x 3
Out of the shadows x 2 (thinking of 3)
Leap of the Clouds x 2 (never a dead card, movement or AC)
Sneak x 2
Alert x 3
Backstab x 3
Zanias Trichery x 1

Items:
Cloudracer x 3
Altu's Cloack x 1
Viper's Tongue x 3
Gloves of Mishief x 1

As you see it's a little over 50 is going to be 52 or 54, pls comments.

I would suggest something closer to this:

Taisia
x2 Keredwin
x3 Anselem

Characters
x3 Thunic Wyvern (alternate start with better AC, innate movement)
x3 Brine Fiend (also for an alterate start)
x3 Judge Aeacus (Multiwounding always good)
x3 Markappal Ivy (control)
x3 Flesh Golem (Freeks hate this guy)
x2 Ta'anar Tol'son (works for melee and ranged)
x2 Sedayah Rowan (meta against Dwarves and Freeks)

Actions
x3 Leap of the Clouds
x3 Swift (can also be played by Keridwen )
x3 Zaina's Treachery
x3 Misear Diplomacy
x2 Backstab
x2 Cunning Shot

Items
x3 Camel
x2 Cloudracer
x2 Bola
x2 Viper's Tongue

I would start here... You might be asking why no Sneak, or Alert. With nothing to boost skill right now (outside of freeks and Sister Megaria), I have not found it very helpful, even on occasion. Sneak is strong, but I feel confident with 8 skill and Stealth +5... That's just me... Hope this helps! And don't be afraid to make proxies of what you don't have! Goodluck!

How about mobility?

Unless you can play mobility in the opposing army, it's not a solid include for this Rogue... It could be helpful in a pinch, but overall, I do not think it is worth it! Test Mobility out, and see how you like it, and report back! Goodluck!

Thought I'd step in on this thread, because while building my Yahia deck to playtest, it hit me with another addition of SE, that Taisia could be rather interesting.

Jacob Windrider of course. He'll acquire the stealth and rogue class/alignment, not to mention the assassin trait from this warlord. I'll have to look into some of the cards usable from this character.

I know he plays an interesting role in my Yahia deck at the moment, as he is able to cast Phantasmal Killer as well in addition to Yahia when she equips a Wyvernskin Robe.