Trunks & Mai

Future Trunks and Future Mai
until adulthood, a few years after Future Trunks finally defeated the androids and Cell. Together, with the help of Future Trunks' mother, Future Bulma, Future Trunks and Future Mai helped rebuild the world and consoled the survivors of the Androids reign of terror. As they continued to work together, Future Trunks and Future Mai developed romantic feelings for each other but publicly denied their relationship as being anything other than a friendship or partnership. However, time of peace was disrupted when Goku Black appeared and almost destroyed whatever was left of the Earth's population. Future Trunks tried to fight him but failed and was almost killed. After Future Bulma was killed, Future Trunks decided to travel to the past to seek assistance from Vegeta and Goku but when Goku Black tried to stop him, Future Mai risked her life to hold him off and give Future Trunks a chance to escape, despite his protests. Upon arriving in the past, Future Trunks believed Future Mai was killed as a result and mourned her, but when he returned to the future with his father and Goku, he found out that Future Mai was alive and he was greatly relieved to the point he burst into tears. Future Trunks even fed Future Mai a Senzu Bean via mouth-to-mouth. Everyone, with the exemption of Goku, realizes the depth of the affection between Future Trunks and Future Mai, and even Vegeta asks Trunks to flee to the past with Future Mai because of Future Trunks' vow to protect her. When Goku Black and Zamasu are finally defeated, Future Trunks and Future Mai are more relieved they survived together than their world being erased by Zen-Oh. Ultimately, they decide to travel to another timeline to live together, despite being told they will be living with another set of themselves in this timeline but they have no problem with it.

Present Trunks and Present Mai
Trunks and Mai met as children as well (Trunks was twelve and Mai appears to be around the same age). They met during Bulma's birthday party when Mai and the Pilaf group were trying to steal the Dragon Balls. In the movie, Trunks found them and asked what they were doing. However they ran away and Trunks became curious of Mai. To keep her around, Trunks told his mother they were his friends and then said Mai was his girlfriend. Mai however was repulsed by this because she was older than him (as she is chronically much older than him but appears as a child). But she went along with the ruse to get the Dragon Balls. She later held Trunks at gunpoint to get them but everyone thought she was playing around, and even Trunks believed she was acting. She hid from everyone as Beerus fought all the Z Fighters before leaving after fighting Goku. At the end of the movie, Trunks and Mai are sitting together in a tree. In the series, he was curious why was she was at party and offered her food out of pity. She refused but he still gave her food before she and the Pilaf group ran away and Trunks went off to play with Goten

In the series, Trunks and Mai are taking tutor lessons together when Future Trunks appears. Mai immediately becomes infatuated with Future Trunks and Trunks becomes jealous. Based on his reaction, it is implied they may have met sometime before, long enough for him to develop a crush on her. Trunks tries to find ways to impress Mai but is turned down. Trunks then takes out his frustration out at Future Trunks by blaming him for failing to protect Future Mai. Trunks eventually thinks that because Mai is interested in Future Trunks, who is his future counterpart, then Mai likes the present him as well. Trunks' crush on Mai becomes to profound that he will madly blush at her, even when he finally meets Future Mai and is amazed how beautiful she is. When Future Trunks and Future Mai leave for good, Mai tells Trunks she is not sad because she now likes the present Trunks more now, much to his surprise.

Trivia

 * Mai is much older than Trunks; when they first meet, he is 12 and she is chronologically 41 but appears as a child as a result of wishing for her to be young again with the Dragon Balls.
 * Piccolo criticizes Trunks for having a girlfriend at a young age, but Vegeta and Bulma have no problem with it and often allow Mai to stay at their house with Trunks.
 * Because there is no evidence of Bulla marrying, it is highly possible that Trunks and Mai are the true ancestors of Vegeta Jr., which would make him their great-great grandson.