Reckless Cohort checks whether you control another Ally as attacking creatures are declared. At that time, if you donâ€™t, Reckless Cohort attacks if able.
Reckless Cohort can attack if you control another Ally. It just isnâ€™t forced to.
If, during its controllerâ€™s declare attackers step, that player doesnâ€™t control another Ally but Reckless Cohort is tapped, is affected by a spell or ability that says it canâ€™t attack, or hasnâ€™t been under that playerâ€™s control continuously since the turn began (and doesnâ€™t have haste), then it doesnâ€™t attack. If thereâ€™s a cost associated with having Reckless Cohort attack, its controller isnâ€™t forced to pay that cost. If they donâ€™t, Reckless Cohort doesnâ€™t have to attack.