that sounds cute!! he'd probably be an embarrassed little bro who'd tease her right back like calling her old...or making fun of her boobs or smth since he's like. a toddler

he's been at the monastery since he was a kid so when she arrived he would've been like? 13?? and also! rin was out on westerley during ch.3 so if she was in old town at the time he would've definitely come to visit/pester her.