Ok, a good problem to have but I would like input.
I get to promote one of these three heroes, but can’t decide.
Roostly is strong until the opposing tank is gone…and then he is less impressive.
Mica I don’t find overwhelming, but I have Cobalt to pair him with (so a small family bonus)
Bai Yeoung I am a big fan of because I feel like he could slow down Frigg and Odin who are the current flavor of the month.
Odin and Frigg are heroes I am matched against constantly. What say you!
I’m liking Bai here. Roostley is great in a Very Fast event, other than that, meh.
Mica, meh.
I am also seeing a lot of Frigg/Odin on defenses too.
Mica is great, and some would say the best of the Ninjas for defense. Cobalt has synergy with Mica due to the Ninja family bonus. That might be your best choice, depending on what you want to use him for.
Roostly also is great, with tanky stats too. But, like you say, he’s less valuable once the tank is killed.
Bai Yeong is hits too softly to make much of a difference. I see your point about Frigg and Odin, but I doubt he’d make a huge difference.
Mica will protect cobalt.
Ninja fam bonus makes it can dodge tiles or slash attack
Hands down Mica I’d say. I maxed Bai and he/she sucks. The panda’s blind isn’t nearly as strong as advertised. Maybe mine is broken, but it seems to blind at a 50% clip and not 85% like shown on the card.
So, I do have Bai Yeong maxed on my alt account and it works well for me, but SG luck is a weird thing.
I have roostley and bai maxed and emblemed.
Roostley is good for tile damage and defense. Use him as a softener and finisher of low health heros.
Bai is most suited for offense. He is very useful similar to Malosi, his perk is that i can now avoid bringing a healer to purple titans because coupled with joon, titan has blind/miss all the time. Dont forget his elemental link is great as well.
Mica, dont have, i see not much use from him to passive for my taste. though, i hate the ninja family bonus and since you have cobalt they would pair nicely on defense. As yellow lacks good healers him being able to reduce incoming damage very early on, could be helpful, if you have survivability issues.
Out of those 3 you will have i think the most fun with Bai Yeong. His blind may fail some times, but think of probability more like this:
His blind with 75% miss chance is comparable to the chance of pulling 3* hero from portals (nsead of 4* & 5* heros = miss fails) and we pull 3* heros all the ■■■■■■■ time.
I have Bei MAX and I can tell you that I am very happy! He has saved me countless times in (wars, Titans) so I use him often! His blindness to me always works or if not most of the time! With hands down Panda first!
Yours is broken I’d say since it’s 75% not 85% 
But that’s Blind. When it’s hot it’s broken OP…when it’s cold it’s just broken down.
I’ve had all 5 hits miss and I’ve had a game where I had 3 left and they hit all 3.
But they miss a lot more than they hit on mine…you just remember the hits as they usually really hurt you.