I have +Res -SPD Legendary robin, and i'm not sure if -SPD is the second best bane, because banes like -DEF or -RES makes Legendary Robin squishy, and -ATK is weak.
Is SPD a good boon and bane at same time?
Um, yeah, I would say -HP is the best bane, but a Speed bane can be patched up with buffs and QR to guarantee doubles and the denial of doubles from opponents. So I guess you're right, a Speed boon can help her land doubles, a Speed bane can help her boost other stats while maintaining her viability as a decent tank.
She's very versatile with her IV's, I find. If you wanna make her an offensive character, +Spd is really good, and you can sack some of her Def or Res without issue. If you want to make her a tank you can run -Spd or -Atk really without issue. (The adaptive damage and access to flyer buffs means her low attack isn't as much of an issue as it seems.)
I run a +Spd -Res with Fury 3/Vantage/Res Tactic/Def Tactic and she's pretty great. Even with the bane and without any buffs to her defenses she can still tank a hit, and her offenses, especially when buffed, make her a pretty great all-rounder. I don't think her IV's really limit her much, you can build her to her strengths and she's quite amazing.
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