she can build up to it with birthday/christmas/etc, maybe multiple rings
but other than like 'couples/promise rings', apparently the pinky finger is a symbol of 'lovers' in japan too (tho animanga wise i've only seen it like secondhand of ppl teasing the protag or like raising it in a way to ask/gossip if someone does have a lover or not [tho i would've expected it to be some sign language for like 'friends' since doing a 'pinky promise' might be a common thing too])