Mai is a recurring antagonist and the love interest of Trunks in Dragonball Z: Battle of Gods and later on in its serial adaptation Dragonball Super.
Mai is a minion of Emperor Pilaf, a bungling villain who tries and fails numerous times to gather the dragonballs, only to be thwarted by Son Goku and his friends. When Pilaf's crew manage to successfully summon the dragon, Pilaf wishes for youth. This turns Pilaf's gang - Mai included - into pre-teens.
In Battle of Gods, they try to gather the dragonballs again, this time at Bulma's house. When Trunks catches them, he's too distracted by Mai to stop them from stealing a dragonball and running away. Trunks catches them later on and asks Mai to pretend to be his girlfriend to impress everyone.
Mai eventually warms up to Trunks by the end of the film, as they're seen huddled in a tree watching the stars. They've been going steady as of Resurrection F, as Mai tries threatening one of Freeza's minions by mentioning her boyfriend Trunks.
In Dragonball Super Mai's relationship with Trunks isn't given much attention until much later, where it's revealed that Trunks and Mai have been spending more and more time together and have naturally grown attached to one another. In an alternate future, Mai has become the leader of a small resistance fighting against a new entity alongside the now-adult Trunks. This version of Trunks also shows an attraction to Mai and she seems to genuinely care about him as she risked her life to make sure he could get back to the past.