Shiro would probably stare at them for a while as his brain processes the confession for a few minutes, blinking every now and then.

Slowly, it'd hit him and he'd realize what just happened. He'd get real awkward and quiet, red AF in the face.

Before giving them a signature warm grin and taking their hand to hold.

He couldn't reject anyone's confession and would give anyone a chance. Get to know 'em better.

Unless it was Sapphire. Then he'd just immediately burst out laughing and tell her to quit lying and joking with him.