Colt Walker is a rather odd Doll who comes with incredibly strong Damage tiles, combined with a passive skill that debuffs your own team's RoF. In addition to this she also prevents the enemy team from gaining Damage buffs.
She should generally only be used in MGSG echelons as the RoF penalty is absolutely devastating for normal echelons.
Pros
Extremely Strong Tiles
60% Damage is very high.
Prevents Enemy Damage Buffs
Enemy Murats can be a massive problem if left alone, buffing Damage repeatedly as the fight goes on and swiftly causing problems for Armor tanking. Walker prevents the Damage buff from working.
Cons
RoF Debuff, Prevents RoF Buffs
Very unsuitable for standard teams.
Full analysis
* ColtWalker does not cancel out RoF tilebuffs or additive RoF buffs (ex: from SCAR-H) * ColtWalker’s RoF debuff is reapplied every few frames and effectively cannot be cleansed. * ColtWalker’s skill will override AUG’s (and similar dolls’) skills, despite not being traditional RoF buffs.