I think Cat o' the Canals needs one of the following two errata:
1. House Stark Only
2. Change "No attachments except a single Weapon" to "No attachments except a single [stark] or neutral Weapon".
I currently only play standard format and there are three weapons in that format that would make her an out of house monster.
Firstly consider 3 Cat o' the Canals & 3 War Scorpion in a Martell deck. It will cost you 6 gold, less if you use Under One Banner or another reducer, but once it's out you have a freaking Juggernaut! They can't target the character to spoil the attachment as is usually the case. They can only discard the attachment or prevent it's triggered response. With this combo you can defend any challenge with her alone and kill all other participants after it's resolution. They can't stealth you or remove your icons, they can't stop you from defending (Wex Pyke, Cersei Lannister ITE). A PttQ would usually prevent it but Martell are strong on intrigue challenges and targetted card discard fits in quite well too (Confession, Ghost of High Heart, Varys).
Now look at the usefulness of the Cat o' the Canals dupes. Normally dupes are of questionable value because the character can be killed rendering the dupe a dead card. With Cat o' the Canals the dupes become 0-cost military claim soak that cannot be countered, quite useful in Martell. Usually you have weenies or saves to deal with claim soak. Weenies can be killed before claim with various effects and saves can be cancelled which often results in a more valuable character dying. The dupe is a save that can't be cancelled.
Second possibility, use Cat o' the Canals, Driftwood Cudgel & Greyjoy Master at Arms in a Greyjoy deck for slaughter of non-uniques. With the Greyjoy Master at Arms in play the Cudgel won't be going anywhere ... or at least not for long.
Thirdly, Cat o' the Canals & Aegon's Blade in a Targaryen deck. Now you have an untouchable tricon character with 3 STR, Stealth & Renown. You can never remove that renown power (it's LCG legal too). Hmmm, now that I think about it there are a fair few cards that would allow you to transfer power to her where it's nice and safe. Add Cat o' the Canals to an Infamy deck with Wardens of the Reach and you can make all of your power untouchable. So I guess another errata is in order: "power may not be placed on this card".
Ok, so I think she should get the following two errata:
1. Change "No attachments except a single Weapon" to "No attachments except a single [stark] or neutral Weapon".
2. "Power may not be placed on this card"
She would still be quite formidable in a Stark deck with these changes, Ice & Cold Steel combo nicely with her. Greyjoy unopposed hate her too.
What do you guys think?