If you have an item that reduces damage and you take damage during a check, then it's usable as long as you didn't already play an item during that check.
The "one card per type per character" restriction is also "per step" not "per check" so you can reduce damage with an item even if you already modified the roll with an item and/or determined your die with an item.
It is per step of an encounter (and attempting the check is one step of the encounter), not step of a check. So you can't play an item to reduce damage from failing a check if you already played an item on that check. And the rule is also mentioned as “per check“ for those situations when checks happen outside of encounters (a check to close a location or to recharge a spell like cure).
RotR Rulebook p11 wrote:
Remember that each player may not play more than 1 card of each type or activate any 1 power more than once during each check.
S&S Rulebook p9 wrote:
A specific card’s power may only be used once per
check or step.
And the game is very careful not to call the parts of a check "steps". Steps refers to the steps of the turn and the steps of an encounter.