When you add up all the value Martyr gives you - 4 damage and 3 blocks - it's a phenomenal card. The problem is she's a little slow and situational, especially in comparison to some of the great options in the same aspect like Iron Fist, Polaris, and Luke Cage. Each of these gives you about the same value but without any extra work. The Guardian trait is fine, and you can definitely build a Knowhere protection deck or go multiplayer with Blaze of Glory. Considering other options, especially if you are doing an Avengers or X-Men trait deck, Martyr rarely makes the cut for me, but if your collection is limited or you just want to try someone new, you could certainly do much worse.

@stretch22's review mentions that Martyr offers great value, but a bit slowly. One way to accelerate that value is by pairing her with Hangar Bay. Even then she needs some help because the base 2 ATK isn't going to defeat a ton of minions. She's definitely got scope for a Voltron build, I think.